The United States uses a federal system of government. In theory, this means that the States hold general police power, while the national government only has power over crime between different states, crimes affecting interstate commerce, criminals who cross state lines, or crime committed by members of the national armed forces, or crime that crosses the international borders.
Hang with us. That was the easy part.
The reality is: the meaning of law is whatever the "court of the day" says it is. This has allowed the national police agencies a huge amount of power, far beyond anything imagined by the folks who drafted the Constitution.
Stay tuned for what "court of the day" means.
The word 'Federal' is used by most people to mean 'National' most of the time in the USA, although technically the word 'Federal' means the relationship between the States and the national government. Just assume 'Federal' means 'National', i.e. controlled by the national government. Unless otherwise specified, Federal law enforcement agencies are under the Department of Justice, which is part of the Executive Branch of government. This can be a problem when the person being sought is a member of the Executive Branch.
We are about to dive into the topic of police agencies. Unlike some police forces, pretty much all of the law enforcement agencies in the US issue firearms to their officers. Every member of every agency listed after this has a gun strapped on. All of them. Track that.
While marked police cars in most countries run the gamut from minicars to near-exotic sports models, American cops favor big, preferably rear-drive and V8-powered sedans; for the past decade more than ninety percent have been Ford Crown Victorias (Police Interceptor model)- Everybody Owns a Ford as almost literal Truth in Television. However, in recent years, the new Dodge Charger has become a real competitor to the Crown Vic, due to it being more powerful, maneuverable, comfortable, and notably, not a 30-year-old design. Also, in many northern states, the Chevy Impala have become a common sight, due to the fact that it has front-wheel drive - not as Badass as rear-wheel drive, but much more useful for driving in six inches or more of snow. Oh, and it's also more fuel efficient than the heavy Crown Vic and the big-engined Charger. Currently, the Charger is used mostly for highway patrol duties, while the Crown Vic and the Impala are used for nearly everything else. The Crown Victoria is being phased out after the 2012 model year, however, and replaced by a new-model Ford Taurus. Some K9 units (not that one) use SUVs instead. Since a K9 team is one human officer and one trained police dog, they need a little more legroom. Those vehicles normally have the same paint job as regular police cars, but with an added silhouette of a dog's head.
Normally, the law and police have power from the State in which they are in. State law authorizes local and city governments to form police departments and establishes their jurisdiction. Thus, most policing is from the city and county police forces.
City police forces (the New York City Police Department, the Los Angeles Police Department, etc.) handle most crimes in their cities. They are headed by a Commissioner, Chief, or Superintendent, who is usually appointed by the Mayor. (What constitutes "handling crime" can be a matter of some debate in cities that melt down in social/political chaos, like Sanford, Baltimore, or Ferguson.)
The Sheriff's office or department is the typical name for a county-level local police agency, and handles crimes where people flee between cities, or where the crimes take place in areas not incorporated into a municipality below the county level. The Sheriff is usually an elected office. Most of the Sheriff's force are deputies, which are hired positions. Sheriffs in the Deep South have a stereotype of being racist hicks, prone to pulling people over for "driving while black". Sheriffs or county marshals almost always serve as the primary enforcement arm of the local courts, serving warrants, writs, and other court orders, and running the local jail (in many places, these roles may be all that the sheriff is responsible for). Sheriff are also commonly designated in many places by law as the county coroner, often a position called the "Sheriff-Coroner," and has responsibility for finding and properly handling unclaimed dead bodies in the county (and, very often, also running the local medical examiner's office). Sheriff's offices may also handle specialized and expensive law enforcement resources, like criminalistics laboratories, helicopters, search and rescue teams, or SWAT teams and make these available to local police agencies too small to have their own versions of these.
In some cases multiple cities may form metropolitan police departments that merge city- and county-level agencies into a single department with county-wide jurisdiction (the Las Vegas Metropolitan Police Department is an example, having been created by a merger of the Clark County Sheriff's Office and most of the local police departments in the Vegas metropolitan area), or smaller cities may contract-out police services to larger neighbors including the Sheriff. For rural areas not large enough for their own police force or for crimes taking place on state highways, the state police or highway patrol will often handle it.
Of course, this doesn't apply everywhere. In some places the police have completely supplanted the Sheriff as the primary law enforcement officer, in which case the Sheriff may shift into a role similar to the U.S. Marshals (see below) as an enforcement arm of the courts...or they may have had those duties assigned to the Staties or another agency, in which case they don't really serve much of a purpose but continue to stick around anyway. (9 out of 10 New Yorkers won't know that the New York City Sheriff's Office still exists, let alone what they do.) Naturally, this varies depending on where you are, because if there is one thing Americans love, it is for every little town to have their own way of doing things and their own set of laws. They're also often given jobs city police forces find too difficult or that don't fall clearly into any one agency's jurisdiction; e.g. the Los Angeles Sherriff's Department polices Southern California's passenger railways, including the LA subway.
Wait! We drifted off 'simple' for a second.
As far as an example of enforcement goes, the hierarchy for Beverly Hills in California is like this: Beverly Hills, then LA County Sheriff, then California; unless the miscreant flees from Beverly Hills to Malibu via non-county or state roadways. Then it is Beverly Hills, LAPD, California State, then Los Angeles County Sheriff (via contract with Malibu City). State and federal come later, depending on what the miscreant was up to.
Not simple enough? Put it this way: If you mess up in Beverly Hills and flee in any direction more than four miles, there will be up to six agencies trying to get an arrest statistic with you as the number.
State troopers are responsible for apprehending criminals on State highways or across county lines, and also for investigating crimes involving state property and protecting state-owned facilities. In most of the States, the "Staties" are about traffic tickets. Most of the Interstate highways are patrolled by Staties. They can be called the State Police, the Highway Patrol, the Department of Public Safety, or something vulgar. (People don't like getting tickets.)
The State Police can also be called in if there is a conflict of interest with the local police force or accusations of corruption. The State cops, in most States, will then crawl directly up the butts of whoever it is further down the chain that is not giving them a cut of the proceeds.
Naturally, this doesn't apply when the Staties are actually the heroes (e.g., CHiPs), in which case expect a lot of car chases.
A common situation in some smaller states is to give the Staties primary responsibility for most homicide investigations. For example, the Massachusetts State Police handles all homicides occurring outside of Boston, Springfield, and Worcester (sorry, Jesse Stone). In Maine, the Staties investigate homicides occurring outside of Bangor, Portland, and Cabot Cove.
Some states have multiple statewide police agencies. For example, the Texas Highway Patrol handles the traffic tickets and the car chases with the aid of cute helicopter pilots, while the Texas Rangers are in charge of investigations and roundhouse kicks. Up until the mid-1990s, California had a State Police separate from the Highway Patrol. The small agency guarded the governor and patroled state parks, DMV offices, etc. The Highway Patrol has assumed those duties. Most states typically also have separate fish and game agencies intended to protect the state's wildlife and enforce hunting, fishing, and conservation laws.
One often overlooked but major type of law enforcement agency that also operates at the state level is the corrections department that operates the state's prisons. Obviously, they have jurisdiction over prisons, which have the highest concentration of criminals, but these departments often also are responsible for managing convicted felons on probation or parole, and investigating criminal activities that are taking place both inside and outside prisons (e.g.: gang activity). This makes these very active and busy agencies, and often surprisingly large (the California Department of Corrections and Rehabilitation, for example, is actually the second-largest law enforcement agency in the United States after the NYPD, not that most anyone would know this).
For National laws, the Federal Bureau of Investigation (FBI) is the primary investigative arm of the United States Department of Justice (DOJ), serving as both the principal federal criminal investigative body and a domestic intelligence agency. Note that although they are the "investigative arm" of the DOJ, they don't answer to them. The Director of the FBI answers to the Director of National Intelligence and to the President of the United States.
It was founded in 1908 and became famous in the 1930s for its battles with organized crime. During World War II it outwitted German Intelligence and nabbed a number of spies attempting to penetrate the United States.
For nearly 50 years the FBI was headed by J. Edgar Hoover, who blurred the line between effective administrator and paranoid tyrant. He tapped phones and assembled files on...well, we can't really be sure about how many people he was spying on, since the files were all destroyed after his death. He was also supposedly a crossdresser.
In general the FBI's jurisdiction often has it going after the "big fish" - Mafia families and the like. They usually don't bother with the small fry, leaving the local police to handle them, which is just as well, since they don't have jurisdiction anyway. The FBI is however heavily reliant on local police departments for knowledge of their own jurisdictions and often manpower. The Bureau typically has more money and resources than it has people in a particular city or town.
Since copyright falls under federal law, the FBI puts warnings that they will raid copyright infringers and slap them with heavy fines and prison sentences on every videotape and DVD sold in America. How much they actually do this depends upon the film industry's
bribes campaign contributions to Congress this term.
The FBI also may be called in to investigate local and State police if there is a conflict of interest or accusations of corruption. National law gives them the power to investigate if anyone's "Civil Rights" are violated. Since one of the rights is the right to a fair trial, this automatically covers corruption or brutality.
The FBI is automatically called in if a crime crosses state lines, or in special cases for crimes that do not but that fall under federal jurisdiction for one reason or another (there are over 200 categories of such crimes). A federal crime typically involves something that crosses state lines or involves multiple states or interstate commerce, or one that interferes directly with the federal government's business—assaulting a federal employee in official business, crimes on federal property, destruction of mailboxes... what? You heard me, that's a federal matter. Good luck actually getting the FBI to come investigate, but legally it is their problem.
- Although they do investigate serial mailbox destruction as it is a sign of tampering the mail and crimes like identity theft. Also since Banks are federally insured, they investigate Bank robberies and white collar crimes associated with them.
- Additionally, they are allowed to investigate any crime which uses interstate communications. Recently it has become involved in such matters as the investigation of Cybercrime.
- And as far as mailboxes, most people don't realize that the Post Office has its own police force so chances are, they handle it first before passing it on to the FBI.
Kidnapping, because a victim might have been taken across state lines, also is within the FBI's jurisdiction.
Historically, they were known as "G-Men" (Government-Men). Some would deem shouting "FBI! Put your hands where I can see them!" as a rather cool thing to say.
There are quite a few people (including Americans) who tend to conflate the FBI with the CIA, since they're both major federal goverment bodies that are supposed to stop "bad guys", can cause govermental paranoia, and have three letter acronyms. The CIA doesn't deal with crime at all, however; they spy on foreign nations instead. The FBI does have responsibility to catch foreign spies in the US, but they don't have any James Bond-types to do it with.
- During WWII, G-Men operated undercover in Latin America keeping tabs on the Germans (remember Cary Grant in Notorious). The CIA took over this function after 1947 (but probably became less focused on the Germans).
- Aside from the fact that having too many Men in Black crawling around in the system is dangerous, the CIA will often not want to follow normal police procedure. If an enemy asset is arrested then the enemy will know he has disappeared. And of course if he is arraigned then they have a big headache of convicting him without revealing evidence. Whereas a spy agency can follow him to bigger fish or lean on him or just make sure he steals the information they want him to.
The United States Marshals Service (USMS) is the enforcement arm of the federal courts, responsible for serving warrants and other court orders, apprehending wanted fugitives, providing protection for the federal judiciary, transporting federal prisoners, protecting endangered federal witnesses and managing assets seized from criminal enterprises. The US Marshal's Service supposedly is responsible for a narrow majority, about 55%, of all arrests made by federal law enforcement.
For more information, and a list of fictional uses, see US Marshal.
The guys in who guard the President, Vice President and other officials, wearing Sinister Shades doing it. The United States Secret Service (USSS) also investigates financial crimes such as counterfeiting, credit card fraud, computer crimes, etc., but aren't terribly well known for that, although that was their original role when they were created in 1865. They didn't get their protection role until 1901, after the assassination of President William McKinley. Formerly under the Treasury Department, they were moved to Homeland Security post-9/11. Since then, the Secret Service is also tasked with handling security for particular "National Special Security Events," like political conventions, State of the Union speeches, even the Olympics, that might be a target for terrorists.
Who gets Secret Service protection is controlled by Congress. Former Presidents now get it for 10 years after they leave office and major Presidential candidates can get it fairly early on in the primary. Barack Obama currently holds the record for receiving Secret Service protection the earliest of any candidate for President. If you guessed that the color of his skin has something to do with this, you win a cookie.
However, the Secret Service also provides guards to protect the White House. They wear uniforms and carry firearms the same as any other police officer, and have patrol cars for traveling near the White House, and are known as the "Secret Service, Uniformed Division." These guys also travel along with the President providing additional security and firepower as part of a "Counter-Assault Team," basically a SWAT team that exists to fight off an organized, armed attack on the President. Among their toys, they have an SUV that has a frickin' pop-up minigun, presumably as a last line of defense to cut down attackers, along with half of whatever city the President happens to be in at the time. See link here.
The USSS has the best rate of capturing criminals of any American law-enforcement organization. Due to the nature of counterfeiting, it is a crime for losers (your odds of being caught if you decide to do it in a big way are over 90%), unless you are an actual enemy nation. (Germany started counterfeiting the dollar in WWII after they had their fill of fake pound notes. Modern-day counterfeiters include Iran and North Korea.) They have, by law, access to the cooperation of any and every other part of the Federal government, including the military, in fulfilling their protective duties.
The Bureau of Alcohol, Tobacco, Firearms and Explosives (abbreviated ATF or BATFE) handles Exactly What It Says on the Tin. One of its predecessor agencies, the Bureau of Prohibition, was home to Eliot Ness's "Untouchables."
Started out in the Treasury Department (hence references to Ness & co as "T-Men" in stories, or at least in Crossword Puzzles), but currently under DOJ. (Somehow "J-Men" hasn't really caught on.)
The ATF has been a highly controversial agency, in part for its role in the middle of American Gun Politics, as well as several colossal blunders over the years (including their involvement in the Ruby Ridge and Waco sieges in the 1990s, as well as, more recently, the Operation "Fast and Furious" gunrunning scandal). Cynical observers of US politics have observed however that keeping the agency around, in a weakened and discredited state, has been more politically useful to gun rights proponents, such as the NRA, than seriously reforming the agency or abolishing it and giving its duties to another agency, like the FBI.
The Drug Enforcement Administration (DEA) is also Exactly What It Says on the Tin. The DEA is unique in that it also administers and tracks doctors' authority to prescribe legal drugs as well as both drug trafficking inside the USA and overt and covert actions in foreign nations. It is the only organization besides the CIA that generally does undercover actions in other nations, and is the only organization period that can do that and work domestic issues. It is the only civilian agency anywhere in the world that flies helicopter gunships. Also under DOJ. DEA does a lot of work on "source control" of precursor chemicals used to make or refine drugs, as well as a lot of undercover work.
In fiction, these guys have a strong chance of going rogue because of all the drug money flying around.
Customs and Immigration
U.S. Immigration and Customs Enforcement (ICE) is also Exactly What It Says on the Tin. It used to be a few separate agencies, but they've been merged, split, and shuffled around a lot in the last few years. Right now they're in Homeland Security, but check back in a couple minutes. It may also be referred to by Spanish-speakers (and Spanish-speaking characters) as "La Migra", a nickname inherited from the Immigration and Naturalization Service. The previous agency, the Immigration and Naturalization Service (INS), was abolished with the passage of the Homeland Security Act in 2002. It is, however, a Discredited Trope since many works of fiction continue to use the name.
The Border Patrol is actually part of Customs and Border Protection (CBP), an agency that's separate from ICE because they have slightly different missions CBP works mainly at the border and at border crossings. They are also charged with preventing illegal things from crossing the border whereas ICE is responsible for dealing with contraband and illegal entrants after they have crossed.
The U.S. Coast Guard used to be part of the Department of Transportation but after 9/11 was moved to the Department of Homeland Security. Originally, the Coast Guard was a Treasury Department agency concerned with stopping smugglers. Its additional duties became ocean/waterways safety and rescue, as well as checking shipping. During wartime, the Coast Guard would be moved to the Department of Defense (but would maintain its status as an independent service) and would also handle coastal and waterway defense. Since the "War On Drugs", the Coast Guard began to change and now spends 95% of their efforts in drug interdiction.
US law means that anything between the coast and 3 nautical miles (5.6 km) out is state territory, while beyond that out to international waters is federal territory.
Coast Guardsmen also serve on Navy and Marine ships in LEDETs (Law Enforcement DETatchments) to handle any civilian-type boardings and such. The Navy and Marines both act as if subject to Posse Comitatus, while the CG is not and does not. Port Security Units are also deployed just about everywhere as, well, Port Security.
The National Guard
The National Guard aren't really a police force, more reservists for the Yanks With Tanks. Each state has an Air National Guard, who play a part in US air defense and get to fly F-16s. 20 Presidents have been in the National Guard at some point in their lives.
The main role in terms of law enforcement is in the field of riot control. The Posse Comitatus Act of 1878 prohibits the use of the regular military in domestic law enforcement, but does allow the National Guard to be used. The Guard's other main role has nothing to do with law enforcement at all—they're typically called out for disaster relief operations when a state gets hit by something really nasty.
The National Guard is run by state and is under the authority of the Governor of that state. The President can, however, "federalize" a state National Guard, placing them under his or her control. The most notable case was in 1957, where Arkansas National Guard troopers were taken into federal command to enforce racial desegregation in Little Rock schools.
He is vengeance. He is the night. He has no jurisdiction.
However, any individual, anywhere in the United States, including a civilian, may arrest a person who commits a felony in their presence. Usually they can also arrest for a misdemeanor committed in their presence as well. Of course, some random civilian telling you to "stop right there!" lacks the oomph that hearing it from a person with a badge does. Civilian's arrests are nearly always by would-be victims or witnesses detaining criminals at gun point or wrestling hold till the cops show up. An ancient right going back to the days of King John and the Magna Carta (note to British readers: you used to have this too, but now you don't even have the right to fight back against an attacker) the rise of arrests by civilians has dramatically increased with the rise of shall issue and automatic concealed carry in the 1990s onward. This number is dramatically smaller in New York, New Jersey, Illinois, Hawaii, DC and most of California  where the legislature stubbornly clings to may issue or effective no issue. Those states are also where most TV and movies are set/written, so don't expect to see it on the small or silver screen too often.
Naval Criminal Investigative Service. Half civilian, this organization investigates crimes related to the US Navy and Marine Corps - any death on a vessel, aircraft or facility.
NCIS is a primarily civilian agency that reports directly to the Secretary of the Navy, who is a civilian appointed by the President, and is thus outside both the Navy and Marine Corps chain of command. However, NCIS works closely with Marine Corps and Navy personnel serving as military police and naval security forces, as well as with civilians who serve as law enforcement officers at some naval installations. Obviously, due to its relationship with the naval service, it frequently operates abroad as well, often performing investigations and security evaluations of foreign ports that US ships might call to.
Despite the name, they don't work for the U.S. Marshals. Look, if you're this far into Useful Notes on America and you were still expecting something to make sense, it's really your own fault at this point. The Federal Air Marshal Service, commonly referred to as the Air Marshals, are a federal law enforcement agency supervised by the Transportation Security Administration, or TSA. They were actually founded in 1968 and existed as a very minor subset of Customs and Immigration. Then, one day, all that changed. Go on, guess the day. I'll give you a hint: It was in the early fall of 2001.
After 9/11 the FAMS was massively expanded and moved into the newly created TSA. They carry concealed weapons and sit on board regular flights while posing as normal passengers. Unlike most branches of law enforcement, they act pre-emptively. Air Marshals go where no crime has been committed, so that they will be in place to respond should anything (like, oh let's say, 3 or 4 men with box cutters attempting to take control of the plane) occur.
Other police forces
Many government agencies (federal and state) have their own police forces patrolling their own property or dealing with criminal activities involving their own activities. In one particularly absurd example, the corner of First Street and East Capitol in DC sits between three government buildings (the Capitol, the Supreme Court, and the Library of Congress), each of which has its own police force separate from the DC Metropolitan Police, who patrol the intersection itself. Likewise the recent purchase of sawed-off shotguns by the Department of Education (for their fraud investigators) has sparked reactions ranging from humorous to paranoid. Some non-governmental entities, like universities and railroads, have also been given authority to establish forces on their properties with full police power. Campus cops never really appear in fiction unless it's as the butt of a joke but you have to learn the trade somewhere.
As the work of such a limited police force won't be terribly interesting, if they show up in fiction, it'll probably be to get in the way.
- California carry permits are left to the discretion of the local sheriff. Most fall into the extremes of red sheriffs who treat it as shall issue and blue ones that approve permits only for important and/or rich people.