Deadpool (Wade Wilson) is one of the anti-heroes of the Marvel Universe, nicknamed "the Merc with a Mouth" (one of his "powers" is actually that his constant chatter and quips can distract and unnerve foes. No, really). He appears in X-Force and several other titles, has had his own run of comics, and most recently had a 50-issue run with Cable (son of Scott Summers and Madeline Pryor) in Cable & Deadpool. He's slated to have a new solo run sometime this year.
His backstory, in short, is that he was a mercenary, got cancer, went into Weapon X (where Wolverine was also experimented on) to try to get cured, endured experimental science-stuff, and got a form of Wolvie's healing factor. SADLY, while it stopped the cancer it made him slightly unhinged (think complete ADD plus memory problems plus mood shifts plus...) and also caused his skin and outer appearance to be grossly deformed. Also they tortured him and stuff at The Workshop (for failed Weapon X experiments) and the reason he's called "Deadpool" is because they used to have a dead pool running on which inmates would die next at The Workshop, and he was King of the Deadpool because they thought he'd never die. Eventually he escaped and went back to being a merc. He's super-skilled in martial arts and weaponry, and he's absolutely hilarious. SNARKY, even. :) Also, he actually defeats Wolverine at one point (when Wolverine is already really, really weak). There's more to the story than that, (like Zoe and the LLL firm) but that's the basic deal.
He's also appealing/interesting because he wants to be a "hero," (sometimes) but can never quite make it. So the comic is very dark sometimes, but almost always retains the humor of his character at the same time.
And now, some fun examples of Deadpool's charm:
Deadpool gearing up for a job.
Daredevil/Deadpool Annual (a great story arc here)
Wade makes a law joke! ♥
Dear Deadpool, the letters page where Deadpool breaks the fourth wall and answers questions from readers. It won an award for best Letters page.
Deadpool "Respect" thread - some good scans.
In related things, Iron Man - interview with Robert Downey Jr. (he uses the word "nerdgasm." HEE.)
an article on upcoming Marvel movies
newly announced release dates for some of them.
the official Marvel movies page (look, there's going to be a new X-Men animated thing)
a list of all the films based on Marvel comics.