Vampires that sparkle are firmly in the romance category
I've really found that fantasy is often more easily defined than science fiction. You have adventure fantasy and epic fantasy, but you know both are fantasy. The issue really comes when an author deicides to push the genre and inncorperate elements of another genre into whatever one they're working on. While far from a bad thing, this does cause a problem with clearly identifying where a book belongs. I did learn in high school that science fiction is less about technology being present and more about examining the consequences of human actions on a large scale. Obviously this also doesn't cover every book in the genre, and it also includes books outside the science fiction genre as well.
I guess this comes down to the nature of genre itself... Which is really just a way to sort the books on our shelves and to find an area to focus on.