"It's 100% manly."
You know you got really hyped up over the Deadpool trailer. Because Deadpool's life is dope and he does dope shit (wait... no that's Kanye West). Anyway, the movie isn't coming out until 2016, so you have a few months to get your act together and learn everything you need to know about the character before you see him in glorious live-action.
This card will give you the basics. Don't worry, I'll be gentle.
He was actually a knockoff of a different character.
Deadpool was one of Rob Liefeld's early creations, "thinly-veiled reproduction of another character", Slade Wilson aka Deathstroke from the Teen Titans comics. Deadpool's real identity, Wade Wilson, is an reference to that character.
This is Liefeld's usual standard, just FYI.
Deadpool was a villain for a little while, and it wasn't until Joe Kelly and artist Ed McGuinness started working on the Deadpool ongoing series in 1997 that the character started to take shape, and become the merc with a mouth that we know and love today.
Deadpool became an anti-hero.
He has no moral compass, but he's trying to be a hero and do good without one anyway. Kelly wrote a snarky character, full of lightheartedness that belied the frequently grim stories. In a lot of ways, Deadpool was a way to satirically critique the superhero genre. And after the series was wrapped up (after two thwarted cancellations), other writers and fans began to carry on that tradition with the character.
Deadpool knows he's in a comic book.
Deadpool's powers aren't limited to his ability to heal from pretty much anything (the last time his head was chopped off, he grew a new one). He's a completely self-aware character. As you can see in the image above, he remembers the last issue he was in with Bullseye. That's the kind of trivia lots of people would have to Google.
The other two images are Christopher Priest's contribution to the Deadpool cannon, which continued the tradition of breaking the fourth wall. Gail Simone's work on the character ensured that the idea was here to stay.
Since then, other artists have added their own spin to the character.
But that should give you the basics of how Deadpool developed (if you want to start reading the comics and aren't sure where to start, this article will help you out). Off the record though, fans have had a variety of responses to the character. You're familiar enough with the canon now that it's time to talk about fanon. Which I mill make another card for... later!