4chan is an English-language image board that was created in 2003 by Christopher "moot" Poole as an American version of the Japanese Futaba Channel (2chan). He translated the source code of the Japanese image board to English, then started up what has been the most famous (and infamous) English language image board of all time.
4chan was originally created for the discussion of Anime, but has grown into a collection of image boards dedicated to all sorts of topics. Video Games (at one point the fastest board on the site before it got broken up into separate boards in 2020), TV series and Films, to more niche topics such as Flash animations, cooking, and creepypastas, you name it, the site likely has a board for that. Posting is anonymous, though nicknames can be used. Anons however don't like users who use nicknames ("namefags"), so expect to be quickly shitposted off the site for using one of those unless you've got something really major on your hands.
However, it isn't just images 4chan is known for. 4chan has been the birthplace of lots of internet trolling, contributed a lot to the culture and formation of Encyclopedia Dramatica (both its former and current incarnations), started a ton of internet memes, and has been the center of a ridiculous amount of publicity, both good and bad. By 2015 however, a changing of a guard came when "moot," following a period of drama and controversy finally resigned from running 4chan and now working for Google. Time will tell how this will affect the site in the long run.
- Anime and Manga: Covered by the oldest non-random board on the site, /a/. Unlike most other places dedicated to anime, /a/ enjoys fanservice and beautiful women dressed as skimpily as possible, Moe works and in some cases even incestuous love (but only if "wincest" between hot siblings, parent-child incest is regarded as Squick). It also hosts a fairly devoted retro anime community which tends to like just about everything 80s and 90s anime, with some 70s stuff thrown in for good measure.
- Defector From Decadence: 8chan, after a fashion. Given that a good portion of its users were originally from 4chan, who migrated for one reason or another. The site went down after the Christchurch attack in 2019 and is now but a shadow of it's former self, with a definite successor emerging only in 2022 in the form of frenschan. Note that this isn't the first migration 4chan anons undertook - one of the first bigger ones was in 2006 when many users from /b/, the (in)famous random board, left the site for 7chan and 420chan (both defunct as of this writing) after moot decided to ban the much beloved institution of raiding other sites for the lulz.
- Ecchi: /e/ is the board for this. Also frequently posted to /a/ as there are no explicit restrictions of ecchi content to /e/.
- GIFT: There is a very good reason this site is called the "asshole of the internet".
- Lolicon: And Shotacon. Though theoretically banned there for some years, the site is still overflowing with them, which push the rules the site as far as possible to keep their community going on inside it. There are also the ironic ones who simulate it to try to push off "normies" (aka people disgusted by it) off discussions.
- 4chan also notably popularized the concept of "straight shota" or /ss/, which is essentially sex between a boy (usually prepubescent) and an of-age woman. Yeah.
- Hentai: Two boards just for drawn sex - /h/ for Japanese hentai, /aco/ for sexually explicit pictures of Western cartoons. /d/ is a borderline case, as it features non traditional porn.
- Humongous Mecha: One of the oldest boards on the site, /m/, is dedicated purely to these. Japanese, American, you name it, if it has a giant motherfuckin' robot, it's allowed on /m/. Spaceship-based shows are also allowed there, and it's not unusual to have Yamato and Harlock threads, since the Japanese too consider them mechanical.
- Incest: Depends on the kind of incestuous relationships. Brother-Sister Incest and Twincest are generally seen as "win" and are almost universally supported. 4chan is notable for creating the Western incest fandom in anime, with titles such as Kiss X Sis, Please Twins! and Oreimo still being held in fairly high regard, sometimes ironically but usually in a honest manner. Parent-child incest more often than not falls into the Squick category.
- Memetic Mutation: Tons of memes have been born due to 4chan. A few of the most famous have to be the Rickroll and Pepe.
- Politically-Incorrect Hero/Politically Incorrect Villain: /pol/ is the most infamous board of 4chan ever since Gamergate kicked off the liberal vs conservative culture war. There is no middle point - either you consider those fellas to be ridiculously funny trolls mocking current trends or you consider them unironic racist scumbags.
- Prank Call: Beginning in early 2011, 4chan organized a prank calling of the Gold and Silver Pawn Shop, home to the popular television show Pawn Stars. The callers would repeatedly ask the employees if they sold Battletoads, a video game for the Nintendo Entertainment System notorious for its difficulty. This call led Rick Harrison, owner of the store, to repeatedly swear and yell at the prank callers, who recorded this and uploaded it to YouTube. This originated several other similar videos of pranksters dialing random establishments and asking about Battletoads.
- Rules of the Internet: Created thanks to 4chan.
- Rule of Funny: /b/ used to be the main lulz board, however as it started to become overrun by porn [s4s] became the main meme board. /r9k/ also contributes to meme production every once in a while, as it's a random board that used to have a rule of "no reposts".
- Troll: Tons of these can be found on 4chan too.
- In the recent years, /pol/'s trolling of Shia LaBeouf's He Will Not Divide Us (or HWNDU, for short) anti-Trump "art installation" has become widely regarded as a classic example of trolls derailing events for the lulz. Prank calls to protesters talking about Nazis, saying to Shia how Hitler did nothing wrong straight into his face, "Kek", "Kekistan" and the sounds of the Italo-Disco song "Shadilay" being a regular occurrence all caused quite the uproar in favor of 4chan's trolling. The protest was shut down twice because of trolling, and the third time it came back as a livestream showing a flag flying at an "unknown location". Cue 4chan finding the flag in one day using contrails from planes, the stars in the night sky and noises of frogs to triangulate the flag down to Greenville, TN. And they replaced the flag with a MAGA hat and Pepe shirt. Naturally Shia was insanely pissed at the turn of events that occurred.
- We ARE Struggling Together: The various boards don't always get along, which can be an understatement.
- Yaoi Fangirl: The Shounen Jump mangas threads with particularly heavy levels of Ho Yay get bombarded with softcore yaoi pics, often derailing the discussions. And there is also /y/, the board dedicated to it.
- Yuri Fanboy: On the other hand, a lot of the Moe show threads are full of fanboys who'd like to see girls French kiss each other. Sometime in 2004, yuri was given it's own board, /u/, to ward off all the porn spam.