I just saw the new Dr. Horrible Comic book or graphic novel. I'm not sure which, because I really don't know.

What is the difference between a comic book and a graphic novel?

I haven't read comic books since I was a kid (until recently thanks to  Middleman and Firefly). When I started performing at Sci Fi Conventions, everything I knew to be a comic book turned out to be a graphic novel.

I'm a bit confused. Any help?