Well, I decided, in order to try to help someone out, that I would do a short ramble on the genre of horror insofar as literature goes. By no means am I an expert, but sometimes just hearing or reading another person's thoughts help.
One of the first things is to decide what, exactly, is meant by horror. Now, we all have our personal horrors: I'm terrified of spiders (thanks to a part-horror book I read when I was a kiddo), a friend who's claustrophobic, another with a fear of heights.... Beyond those, though, we share some frights as human beings. The past haunting the present and stealing from the future the dead clawing their time from the living. If you'd like good, horrifying things to see as examples, read books like "Frankenstein" by Shelley or "The Jewel of Seven Stars" by Stoker or any of Lovecraft's works.
One really good way to describe horror, however, is in comparison to science fiction and fantasy.