Graphic Novels: When is it really just comic books?

So I've been making use of the Ottawa Public Library a lot lately, as I have the time, and it's a nice walk from home. I've discovered that they have a large selection of Graphic Novels, something I always wanted to get into more, and have now had the opportunity to. However, I'm still confused: What's the difference, if any, between a Graphic Novel, and a Comic Book that's just in Hardcover with one storyline (5-10 issues) covered? Because I read a couple, like Watchmen, and one of the Sin City books, but I've also read a bunch of Batman, Daredevil, X-Men, Wolverine, Superman etc.

At what point does it become a novel, and at what point is it still comic books? Either way, I don't think it will stop me, although it's kind of awkward to have to go into the teen fiction section to check some of them out...many thanks to the librarian who actually recommended another one to me instead of look at me like I was a little old for comics...