A brief look at the definition of the Horror Fiction Genre makes us realise that the oldest and strongest emotion of mankind is fear, and the oldest and strongest kind of fear is fear of the unknown.
The Horror fiction genre requires that the reader engages in a conspiracy that agrees to suspend the rules of everyday. Readers must invest strong psychological belief in the literary worlds that are presented.
Horror is an experience of safety and conservatism. Readers are allowed to risk it all from the pleasurable nook of their protected and rule-governed psychological "home" through contrast with the terrifying and the chaotic.
The rise of the Internet has allowed horror authors and fans to create new subsets of the genre. On a lighter note about the genre, there's one thing to remember: The basic idea of horror is that it's got to be horrible.