Ho ho ho! Merry Christmas!
The best known (at least in modern times) mascot of Christmas, developed in the United States as an amalgam of the story of St. Nicholas of Myra and various other seasonal folk heroes, with many aspects provided by the classic poem A Visit From St. Nicholas (popularly known by its first line, 'Twas the Night Before Christmas).
The Santa Claus myth is based largely on the Dutch holiday of "Sinterklaas" (a hastily pronounced "St. Nicholas", who comes down the chimney on the 5th/6 December) and the imagery of the Saint in question carried over to his North Pole incarnation. In the original stories, Sinterklaas was accompanied by black slaves; these have become demons (The Krampus) in German-speaking culture, and friendly elves in the USA. In the Netherlands, the black companions are nowadays portrayed as St. Nicholas' friends and employees. Note that in several countries in Europe, Sinterklaas and Santa Claus are now considered two entirely different characters, each with their own elaborate holiday. It should also be noted that his transition from badass Turkish saint to "jolly old elf" was influenced by another winter gift-giver: Odin. Yes, for some reason, in pre-Christian Europe, the king of the gods would sneak into people's houses on the Winter Solstice and leave gifts for the children, who were expect to leave carrots or oats for Odin's eight-legged horse Sleipnir. During the Christianizing of Europe, this was merged with the story of St. Nicholas giving a father some gold so he wouldn't sell his daughters into prostitution. And that's where Santa comes from.
Santa Claus is universally envisioned as a festively overweight old man with a long white or silver beard, who wears a red suit with white trim (originally a red bishop's robe and camauro) and a matching cap, black boots and a vast black belt worn across his belly. He lives at the North Pole (or in Lapland, or in Spain, or somewhere else depending on your culture - the original St. Nick was Greek, from a city in what is now Turkey) in a large workshop staffed by elves (diminutive commercial-friendly elves, not tall proud Tolkien-type elves) who are often far older than they look, which produces toys year round, and every Christmas Eve he sets out in a flying sleigh pulled by eight reindeer named Dasher, Dancer, Prancer, Vixen, Comet, Cupid, Donner and Blitzen (with an option on a ninth, in the form of Rudolph, on nights with poor visibility), and delivers toys and other gifts to the children of the world out of the improbably roomy sack he carries with him, entering their houses by the chimney, filling their stockings, partaking of whatever food and drink the family left out for him, then leaving how he came in.
Officially, Santa only delivers present to children who have been good and coal to those have been bad, (the idea being that they can burn the coal in their furnaces and avoid freezing over the winter, letting them try again next year; not as much of a raw deal as it might seem to those of us who no longer burn coal ourselves). In even older traditions, he carried a bag of switches for whipping the naughty children. In the Netherlands and Belgium, Sinterklaas is famously accompanied in his work by a servant named Zwarte Piet (Black Pete), which tends to cause headaches with foreigners unfamiliar with the tradition and quite aware of the Unfortunate Implications he represents. (Note that in the Dutch tradition, there is no racist connotation whatsoever to dressing up as a jolly blackface servant and threatening to beat people up. Seriously.) Zwarte Piet himself is a softening of an even earlier tradition in which Saint Nicholas used the services of an enslaved devil. Austria and southern Germany have The Krampus instead. Many other cultures that still look to Santa Claus as an actual saint still include this devil or imagine Santa Claus as doing battle with the devil on Christmas Eve, leading to even more strange reactions from foreigners who wonder what Satan himself is doing in, say, a children's Christmas film.
The traditional explanation for Santa's ability to achieve his annual deliveries is that he is a magical being. However, modern stories dealing with Santa give him access to a combination of magic and supertechnology; some versions even do away with the magic altogether (for example, the Christmas ep of Buzz Lightyear of Star Command has his supertech being used by the villains to commit tons of felonies). In addition, modern depictions of Santa's home have his workshop being a fully mechanized factory run by the elves. A common variation is to have Santa portrayed as not a single magical being, but as a god-like office held by a mortal, such as Ernest Saves Christmas, where Santa is a normal person who spends a large chunk of his (possibly slightly magically lengthened) lifespan as Santa Claus, then passes the title off to someone else.. Surprisingly, despite all that focus on Santa's delivery process, the one aspect of the legend that has rarely, if ever, been called into question is how Santa could accomplish such fits, and still have huge numbers of people (often a numerical majority, even in fiction) refuse to believe that he exists.
Also, for humorous effect he is often portrayed as a cold-hearted tyrant, running his workshop with an iron fist while the elves are a disgruntled workforce. The most vicious recent skewering was at the hands of Futurama, which introduced a futuristic robot-Santa who judged the entire world as naughty except Zoidberg and hunted down the worst offenders every Christmas. Running a close second is the revelation by Anya in Buffy the Vampire Slayer that not only did Santa Claus exist, he was in fact a bloodthirsty demon - perhaps inspired by the devil present in older stories of him.
Since straight portrayals of Santa Claus in fiction have become classics, and re-aired every year, most recent works involving Santa Claus are parodies, deconstructions, or other twists on the legend. Even in strict children's fare, there is always some kind of wry twist on the material: in The Lion, the Witch, and the Wardrobe, for example, Father Christmas muses that he hasn't been to Narnia for many, many years, and proceeds to hand out lethal weaponry such as bows, arrows, and swords to three of the prepubescent protagonists. A major exception is the recent film The Polar Express (although, in all fairness, he's portrayed as less jolly than Santa usually is).
Since the beginning of the twentieth century, there has also been a small movement to explain how Santa came to be, and continues to be. The most prominent backstory for the modern Santa (meaning, not derived from various folklore), comes from L. Frank Baum's (of The Wonderful Wizard of Oz fame) novel, The Life and Adventures of Santa Claus. This story gives Santa a bit of The Lord of the Rings treatment, as there's plenty of strife and battles between the good fairies that raised Santa, and their enemies, a group of rock-monsters. This story has been made into at least two animated films, and continues to be one of the most popular backstories for Santa over 100 years after its first publication. Speaking of Tolkien, he too made his own spin on Santa Claus in The Father Christmas Letters. The Dresden Files has Harry saying that Santa is a fairy. Harry's not willing to summon him either. He'll mess with The Fair Folk, but Santa, no way.
The name "Santa Claus" comes from a Dutch variation of the name "St. Nicholas", "Sinterklaas". It is not, despite what fundamentalist Moral Guardians like to claim, an anagram of Satan. Well, it is, but that's decidedly unintentional. And his last name is spelled C-L-A-U-S, not C-L-A-U-S-E; the latter is part of a sentence or part of a legal contract. This was the basis for a famous Marx Brothers joke. ("There ain't no sanity clause!") The title of the movie The Santa Clause was an intentional pun on this.
An additional note, often in England and Australia Santa Claus is called "Father Christmas" (such as the C.S. Lewis and J.R.R. Tolkien examples above). Although Santa Claus and Father Christmas have picked up many attributes from each other, and are now considered the same person, they were originally distinct characters. Father Christmas was an incarnation of Christmas, particularly of the feasting and drinking, and wore a robe rather than the suit that Santa Claus wears. He was considered to be as old as the first Christmas (unlike St. Nicholas who lived in the fourth century). Examples of Father Christmas from before his merger with Santa Claus can be found in the Ghost of Christmas Present in A Christmas Carol by Dickens, and in the traditional plays of English Mummers.
- Santa Claus is a popular character in Christmas Specials; the most well-known are the stop-motion films by Rankin-Bass. Often times Subbing for Santa is invoked with the main characters of the holiday special.
- Santa Claus as a character is widespread even in countries that aren't Christian, like Japan and China. In Japan, he's called not surprisingly "Santa Kurasu" and in China he's called "Old Man of Christmas".
- In the webcomic *Holiday Wars the Easter Bunny kills Santa Claus and declares war on all the other holidays.
- Darkseid tries to stop him from invading Apokolips every year giving him coal. Darkseid always fails.
- In Axis Powers Hetalia Finland is Santa, following some European Christmas traditions that say that Santa Claus is Finnish.
- Oh, and Deadpool once choked him out with a string of barbed wire while he (Santa) was driving an eighteen-wheeler.
- Which must've been before he got killed by Lobo...
- An automobile version of Santa Claus named Santa Car (a vehicle resembling a Dusenburg) actually appears in storybook based on the film Cars called "Mater Saves Christmas."
- In the Doctor Who 2010 Christmas Special, the Doctor says he is a friend of Santa's, providing a photo of him and Santa at Frank Sinatra's hunting lodge in 1952.
- In Secret of Mana, when the children of the world stop believing in him, Santa tries to use the Mana Seed of Fire to grow a wonderous Christmas Tree that will make them believe, and help him to spread the true meaning of Christmas™ across the world. This... doesn't quite work. The seed's power warps him into the hateful Frost Gigas, and the heores have to fight him to break the curse. Yes, you read that paragraph right. That all happens.
- As mentioned, above, Santa Claus (as Father Christmas) appears in The Lion, the Witch and the Wardrobe to signal that the White Witch's always-winter-never-christmas power is beginning to fade.
- Santa Claus is a friend of Ozma of Oz, and attends her birthday party in The Road to Oz (as a cross-over with L. Frank Baum's book mentioned above).
- It is revealed that there are thousands of Santas in Round the Twist. As Santa #115,302 notes, "It'd take more than one Santa to get down all those chimneys in one night, get real!" Also, Santas have handily evolved claws after hundreds of years of scrambling up chimneys.
- The model for the infamous old man on the lying cover of Phalanx had just come from a Santa shoot.
- In L. Jagi Lamplighter's Prospero Lost, when Miranda is seeking refuge from demons, she is guided into a mall and finds that the Mall Santa really is Father Christmas. Later, she and Mab visit him to use his pool to look for some children. While there, she takes a gift from an elf—usually a foolish thing, but she knows under his roof, it must be safe.
- Harlan Ellison's "Santa Claus vs. S.P.I.D.E.R." reveals that "Kris" is a secret agent in the James Bond (movie) style, his corpulence an armored suit with rocket-assisted jumping, machine guns, and high-pressure hoses up the sleeves to blast out either water or flaming gasoline. His red nose and white beard are disguised weapons, too. He's working against an evil group that's taken over the minds of key U.S. politicians to make them racists, war-mongers, and polluters.
- And lives on in the form of still giving kids who don't deserve toys practical items like tube socks
- Butcher has hinted that he is basically the Fairy King of Winter and a counterpoart of sorts to Mab. One may assume he represents the goodwill and generosity that the harshness of Winter (Mab) brings out in people. Either way, he is a Badass, and Harry knows it.
- As Wiccans will be happy to tell you, he may even be older than that, as the Yule Father, the dating is ambiguous though