Horror fiction is, broadly, fiction intended to scare, unsettle or horrify the reader. Although a good deal of it is about the supernatural, any fiction with a morbid, gruesome, surreal, suspenseful or frightening theme may be termed "horror"; conversely, many stories of the supernatural are not horror.
The horror novel has many antecedents, although the most obvious well-spring is the gothic novel form of Bram Stoker's Dracula, and, less obviously, Mary Wollstonecraft Shelley's Frankenstein. Neither of the foregoing qualify in themselves as horror novels in that their ultimate intention is more one of mood than of shock (and Ms Shelley's is also fundamentally a philosophical novel), that sudden unquantifiable moment when one's flesh writhes. Very few writers are capable of bringing this off, and many modern practitioners of the genre have resorted to progressively greater extremes of violence in order to achieve some sort of effect. Early exponents of the horror form number such luminaries as H.P. Lovecraft and Edgar Allan Poe, who were considered to be masters of the art.
Nevertheless, contemporary writers such as Clive Barker in The Books of Blood and Stephen King in his more considered work, such as Misery, are capable of bringing this off without the grand guignol which characterises much of the current mainstream of this genre.
The rise of the Internet has allowed horror authors and fans to create new subsets of the genre. Numerous web based fanzines have provided a market for both amateur and professional writers which is (for better or for worse) unfettered by the tastes and judgments of the professional publishing houses.