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 (especially Halloween-themed), a disease/virus oubtreak and surrealism, they may be termed "horror".

Its plots often involve the intrusion of an evil force, event, or personage, mainly of supernatural origin, into the everyday world. Some of the most common elements of horror films are ghosts, torture, gore, werewolves, ancient curses, satanism, demons, vicious animals, vampires, cannibals, haunted houses, zombies and masked serial killers. Conversely, many stories of the supernatural do not have to be, and are not, horror.

What's considered to be a horror film has varied from decade to decade. Films that were once called horror, would now be categorised as thrillers (The Silence of the Lambs, Jaws, Psycho, Seven). 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.

The first depictions of supernatural events appear in several of the silent shorts created by film pioneers such as Georges Méliès in the late 1890s, the most notable being his 1896 Le Manoir du diable (aka "The House of the Devil") which is sometimes credited as being the first horror film. Another of his horror projects was 1898's La Caverne maudite (aka "The Cave of the unholy one", literally "the accursed cave"). Japan made early forays into the horror genre with Bake Jizo and Shinin no Sosei, both made in 1898. In 1910, Edison Studios produced the first film version of Frankenstein; thought lost for many years, film collector Alois Felix Dettlaff Sr. found a copy and had a 1993 rerelease.

The early 20th century brought more milestones for the horror genre including the first monster to appear in a full-length horror film, Quasimodo, the hunchback of Notre-Dame who had appeared in Victor Hugo's novel, "Notre-Dame de Paris" (published in 1831). Films featuring Quasimodo included Alice Guy's Esmeralda (1906), The Hunchback (1909), The Love of a Hunchback (1910) and Notre-Dame de Paris (1911).

Many of the earliest feature length 'horror films' were created by German film makers in 1910s and 1920s, during the era of German Expressionist films. Many of these films would significantly influence later Hollywood films. Paul Wegener's The Golem (1915) was seminal; in 1920 Robert Wiene's The Cabinet of Dr. Caligari, with its Expressionist style, would influence film-makers from Orson Welles to Tim Burton and many more for decades. The era also produced the first vampire-themed feature, F. W. Murnau's Nosferatu (1922), an unauthorized adaptation of Bram Stoker's Dracula.

Early Hollywood dramas dabbled in horror themes, including versions of The Hunchback of Notre Dame (1923) and The Monster (1925) both starring Lon Chaney, Sr., the first American horror movie star. Other notable films of the 1920s are: The Lost World (1925); The Phantom Of The Opera (1925); Waxworks (1924); Dr. Jekyll And Mr Hyde (1920); and The Phantom Carriage (1920).

It was in the early 1930s that American film producers, particularly Universal Pictures Co. Inc., popularized the horror film, bringing to the screen a series of successful Gothic features including Dracula (1931) and Frankenstein (1931), some of which blended science fiction films with Gothic horror, such as James Whale's The Invisible Man (1933). Tod Browning, director of Dracula, also made the extremely controversial Freaks based on Spurs by Ted Robbins. Browning's film about a band of circus freaks was so controversial the studio burned about 30 minutes and disowned it. These films, while designed to thrill, also incorporated more serious elements, and were influenced by the German expressionist films of the 1920s. Some actors began to build entire careers in such films, most notably Boris Karloff and Bela Lugosi. Many of the soon-to-be-iconic Universal monster character designs were created by make-up artist Jack Pierce.

Other studios of the day had less spectacular films, but Rouben Mamoulian's Dr. Jekyll and Mr. Hyde (Paramount, 1931) and Michael Curtiz's Mystery of the Wax Museum (Warner Brothers, 1933) were both important horror films.