The word 'genre' comes from the french word for kind or class. Genre is a type or catagory of film such as western, horror , or musical film that follow a set of visual and narrative patterns that are unique within the genre.
While some genres are based on story content (the war film), other are borrowed from literature (comedy, melodrama) or from other media (the musical). Some are performer-based or budget-based (blockbusters), while others are based on artistic status (the art film), location (the Western). It's hard to define a film within one particular genre, since any theme may appear in any genre.
Film theorist David Bordwell asks :"Are animation and documentary films genres or modes? Is the comedy performance a genre? If tragedy and comedy are genres, perhaps then domestic tragedy or comedy is a formula."
Sub-genres illustrate the diversity of cinema, it overlaps across multiple main genres. A fine example for sub-genre are Fritz Lang's 'M' and Hitchcock's 'Psycho.' These movies comes under the category of psycho thrillers, which came from 'thriller' genre. These movies inaugurated a new approach to horror films. Not only did it present the audience with a killer who was obviously human, but also spoke about the deranged minds of a 'psycho killer', who besieges harmless people in these fictions.
A genre can be seen as a shared code between the makers and interpreters of films. But films must adapt to the times when they are produced, they should address various contemporary concerns to which different cross-sections of the audience can relate. Films should not be conveniently typified into a particular group or genre, which gives us stereotyped popular character and plots, and satisfactory predictable resolutions.
In an age of uncertainty, defining a film genre is a hard one. It would be best for critics to leave it open to discussion rather than strict categorization of genres.
Some important film sub-genres:
Black Comedy : A comedy characterized by morbid or grimly satiric humor.(Eg, Dr.Strangelove, Shaun of the Dead, Pulp Fiction)
Film-Noir : The term film noir describes a type of film that is very dark in its outlook. The word noir is French for 'black', and it was during the 1940s that film noir came into its own. The term was coined by film critics just after World War II. Fatalism played an important role in the plots of film noir. The heroes or ant-heroes seemed set on a path that would ultimately lead to their downfall. (Eg, Sunset Blvd., Strangers on a Train)
Bio-Pics : These are sub genre of drama films. These films depict and dramatize the life of an important historical personage from the past or present era (Eg, Gandhi, Kundun, Schindler's List)
SuperNatural : These are sub genres of horror or sci-fi or fantasy films. These films have themes including phantoms, miracles, ghosts, and gods. (Eg, The Shining, The Sixth Sense, The Others)
Psychological Thriller or Horror : It is a sub-genre of broad ranged two genres. Characters in these movies, doesn't rely on physical strength to overcome their enemies but rather are reliant on their mental resources.(Eg, Black Swan, Inception, The Machinist).
List of All Sub-genres from filmsite.org: