OneTusk.com has an interesting essay on genre, specifically the Horror genre and the difference between it and Thrillers. He makes a lot of sense. I don't think I'll be so snobbish as to actually insist on his definitions in casual conversation, but it's good stuff to keep in mind when sitting down to write a horror story:
"I realized that what I'd been mistaking for true horror was simply terror. Horror movies should horrify, thrillers thrill and terrify. There's a subtle difference.
Let's get down to brass tacks for a minute. I looked up 'horrify' and got 'To cause feelings of horror.' Great, thanks. But for horror, I got this:
• An intense, painful feeling of repugnance and fear.
• Intense dislike; abhorrence.
Painful repugnance. Abhorrence. That works. Now, if I watch a Friday the 13th flick, I really don't feel that. Why? Because most slasher flicks are simply thrillers. There's seldom anything horrific--at least in a movie--about people being stalked and killed one at a time. But compare the feelings you get from watching a slasher flick to the feeling of watching John Hurt 'hatch' in the middle of Alien. I can't imagine anybody who didn't twist and turn when they watched that scene unfold."