The term "comic book" arose because the original comic books (created in 1934) were merely reprints of already-published humor comedy strips from the newspapers. Despite their name, comic books are not limited to comedy; in fact, most modern comic books tell stories in a variety of genres other than comedy.
In the United States, the superhero genre dominates the market. In 1938, National Allied editor Vin Sullivan pulled a submission by two Canadian boys named Siegel and Shuster from the slush pile and used it as the cover feature of the first issue of a brand new title. Action Comics #1 was the debut of Superman. Dressed in colorful tights and a cape, Superman became the archetypical model for all of the superheroes who would follow. Action Comics is still being published, having racked up 894 issues as of July of 2010... a number only exceeded by the 1354 issues of Dell Comic's Four Color. (And given that Four Color is no longer being published, it seems certain that Action Comics will eventually catch up and exceed that number.)