Santa Claus is a traditional, secular figure of Christmas good cheer who is 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 and various other seasonal folk heroes, with many aspects provided by the classic poem "A Visit from St. Nicholas."
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. (Note that in several countries in Europe, Sinterklaas and Santa Claus are considered two entirely different characters, each with their own elaborate holiday.)
Santa is traditionally depicted as a festively overweight old man with a beard, who wears a red suit with white trim and a matching cap, black boots and a black belt. He lives at the North Pole in a large workshop staffed by elves which produces toys year round, and every Christmas Eve he sets out in a flying sleigh pulled by eight reindeer and delivers toys and other gifts to the children of the world out of the improbably large 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.
As the most prominent figure associated with Christmas, Santa Claus appears in many Christmas specials and movies, including several produced by Disney. His portrayal in them varies in many ways.
In Santa's Workshop, Santa reads letters from children as he prepares for his big night. This same version of Santa was seen again the following year in The Night Before Christmas as he embarks upon a home filled with children.
In "Miracle at the 34th Precinct", while testing out in his new sleigh, Santa accidentally falls out and lands in Burbank, California, where he loses his memory and hangs out with Fall-Apart Rabbit, who nicknames him Jim.
Santa appears in the episode "Holiday Time". He operates out of the planet North Polaris, where utilizing an interstellar sleigh and a time-manipulation device, delivers presents to all the planets within the Galactic Alliance. However, Emperor Zurg manages to steal the time-manipulation device and seeks to use it to ruin the holiday for everyone. Santa enlists the help of Buzz and his crew, though Buzz doubts that Santa is the real deal.
Santa made his first appearance in this series in "Mickey Saves Santa", in which Mrs. Claus, Mickey and Donald, journey off to Mistletoe Mountain to rescue a stranded Santa. He became an occasional recurring character and played a surprise role in "Choo-Choo Express".
In "I, Brobot", during a "summer run", Santa made use of the Chimney Vator, one of the many systems the O.W.C.A. has in place to transport its agents to and from their lairs around the world. During the ride, he met Agent P. Santa saved Agent P later in the day by catching the electromagnet on his sleigh that Dr. Doofenshmirtz had released from a crane, intending to crush Agent P. For this "summer run", Santa did not require the full complement of reindeer to pull the sleigh.
In Phineas and Ferb Christmas Vacation!, Phineas and friends finish delivering gifts across Danville and find the sauna being used in the rest stop for Santa that Phineas and Ferb built atop the Flynn-Fletcher house. Santa chuckled, seemingly expecting for all of this to happen. He also reads Phineas' letter, saying all Phineas wished for was to be like him. Santa is also Phineas' greatest hero, delivering gifts to everyone in one night, something even he possibly couldn't do. He then takes Phineas' gift with him, thanks the kids for doing Danville for him then leaves. The next morning, Phineas and friends find gifts under their trees, all from Santa ("Phineas and Ferb Christmas Vacation!").
The Prep & Landing specials revolve around a special group of elves that prepare the homes that Santa, codenamed the "Big Guy", visits during the Christmas Eve night. His face is never fully shown onscreen, except from a distance in a few shots in Operation: Secret Santa.