The story of un-super-powered people who dress up as super-heroes and super-villains and then clobber each other, along with clobbering and getting clobbered by plenty of people who don't bother dressing up at all. Interestingly, the movie has the characters as much more sexually active than the comic does. It helps that the actors who play the characters are considerably older than the characters who appear in the comic. It's much easier to see that their hormones are raging.
There was a discussion recently about books versus their movie versions, how movies have to take 200 to 300 pages and squeeze the contents down into an hour and a half movie and how much gets lost in the translation. Kick-Ass 2 was written and drawn with the movie version in mind, but the similar, heavily visual language of the comics and the movies also helped to make the movie greatly resemble the comic.
And, of course, there's a Kick-Ass wikia.