The concept of GENRE

Genre - A set of conventions recognisably usually through iconography familiar narrative mise-en-scene, actors and style of representation.
Horror
Setting - dark, gloomy, spooky, castles, abandoned houses, graveyards.
Characters - Mysterious characters, zombies, ghosts, possessed children.
Themes - issues - Death, haunting, possession. 

1930's - Dracula                                           1970's - Slashers
1940's - Frankenstein/werewolves                1980's - Nightmare on elms street
1960's - Physcos                                          1990's - Teen horror

Genre is always changing in order to keep the audience interested, if films with the same genre kept being produced the audience would become desensitised and the film would no longer have the desired affect.
Genres are not static but constantly renegotiated between industries and audience - a combination of familiar reassurance and new twist.

Genres are creative strategies used by producers to ensure audience identification with a film - a mean of trying to predict risk. Genre is a way or working through myths and fears by: repetition, variation and resolution.