Horror films deal with the viewer's nightmares, hidden worst fears, revulsions and terror of the unknown. Although a good deal of it is about the supernatural, if some films contain a plot about morbidity, serial killers, a disease/virus outbreak and surrealism, they may be termed "horror."
Plots written within the horror genre often involve the intrusion of an evil force, event, or personage, commonly of supernatural origin, into the everyday world. Themes or elements often prevalent in typical horror films include ghosts, torture, gore, werewolves, ancient curses, satanism, demons, vicious animals, vampires, cannibals, haunted houses, zombies and masked serial killers. Conversely, stories of the supernatural are not necessarily always a horror movie as well.
What is considered to be a horror film has varied from decade to decade. These days, the term "horror" is applied to films which display more explicit gore, jump scenes/scares or supernatural content (Wes Craven's New Nightmare, A Tale of Two Sisters, Saw films, The Strangers, The Ring, Session 9).
Early horror movies are largely based on classic literature of the gothic/horror genre, such as Dracula, Frankenstein, The Phantom of the Opera, and Dr. Jekyll and Mr. Hyde. More recent horror films continue to exploit the monsters of literature.