Horror is a genre dedicated to scaring the living shit out of you; and if they can't do that, they will smother you with bared tits.
To most people, there are two types of horror; books and movies. Horror books are written by Stephen King. Rumors persist that there are other horror writers, but they are a bitch to find in the Stephen King section of your local book store.
Horror can trace its roots back to most culture's mythology, which usually seems to focus on scaring the shit out of people rather than actually teaching anything.