Deadpool is a Marvel Comics character created in the early 90's by Fabian Nicieza and Rob Liefeld. He is said to be a rip-off of both Spider-Man and DC's Deathstroke, and for a period around 2010, he was in nearly every comic book Marvel put out.
Deadpool exists in the Marvel Universe as a mercenary, first and foremost. While he occasionally tries to play hero or is used as a villain, it's usually for money or the sheer joy of causing death and destruction. He was created in the early ninteies by Rob Liefeld in order to fight another of Liefeld's creation, the psychic/cyborg/time-traveler/pouch-fancier Cable, and was being paid to do so at the time, though the pair have since become friends. Deadpool uses a mixture of swords, guns, and generalized violence regularly. This makes Deadpool pretty much the epitome of nineties comic books. However, over the years he has grown as a character and become an ever-evolving parody of the comics medium as a whole. This is, in fact, Deadpool's main claim to fame, and while he can still be used as a somewhat serious character, he will always be doing things like breaking the fourth wall.
Deadpool is a reserve member of the Great Lakes Avengers, or whatever they happen to be called at the moment. He also has developed a rotating cast of supporting characters, as well as a few of his own antagonists. He mainly borrows antagonists from other super-heroes, or just fights other super-heroes, and sometimes dinosaurs and space monsters. Really, though, he'll do nearly anything if you pay him enough. He's also on X-Force, in honor of his fallen friend, Cable.
During Deadpool's infamy, he was starring in Deadpool, Deadpool: Merc with a Mouth, Deadpool Team-Up, Deadpool Corps, Deadpool Max, and Deadpool Noir. Nearly all of these have been cancelled, though he's still in a great many things.
Deadpool has appeared in cartoons, though he doesn't really do much in any of them except "Hulk VS" where he fights, in turn, both Wolverine and the Hulk. He lives through this, amazingly.
Deadpool has been in several video games, including the "Marvel Ultimate Alliance" series, and "X-Men Legends II". In all of these, he manages to keep some semblence of his personality intact. Currently, he's a character in "Marvel Vs Capcom 3", where he breaks the fourth wall and does all sorts of Deadpool-type things.
Wade Wilson appeared in the "Wolverine Origins" movie, but then they just sort of turned him into someone who was supposed to be Deadpool but looked like a really shitty "real-life" version of Deadpool, because a movie with an aggressive Canadian who can pop claws out of his hands and features people doing things like teleporting, shooting laser beams out of their eyes, and charging playing cards with energy is so in need of some realism. However, in some cuts of the movie, Deadpool does manage to retain his personality after the end credits. While there's persistant rumors of Deadpool getting his own movie, it's unclear as to whether or not this will actually happen.