
This is an important topic for critics, film makers, audiences, and media theorists. It is very important the way that people chose to watch movies, this is important for the initial marketing of a movie. It as well has academic and as well practical applications. Genres can be as much about the differences between two just as the similarities.
A Functional Approach

In this type of approach the genres in film are perceived as collective expressions in temporary life that strike a particular resonant cord in the audience. Some repetitions that are seen in film genres as well considered repetition of social questions which need to be answered by the audience. They are known as:
- What is frightening, or what possibilities do we fear? (horror films)
- What is criminal, or what are the boundaries of social morality that we must not cross? (gangster films)
- What is morality? (melodramas)
- What is acceptance and belonging? (romantic comedies)
- What is alien? (science fiction)
- What is the future? (science fiction again)
A combination of these two approaches is sometimes considered one of the most successful ones. These questions have as well been repeated throughout the many generations of film and have been tweaked here and there since times have been changing.
Descriptive Approach
The most straight forward way in which analyzing films is the descriptive way. This has to do with the specific categories a film fits into. When analyzing a film you use a thing which is known as paradigm shifts and as well the analyzing of costume, location, character archetypes, shot transitions, or plot content that are seen throughout the film. A thing that is known is that a film can belong to many different categories even though it is said to be in one type of genre.
Sources
http://www.mediaknowall.com/as_alevel/alevel.php?pageID=filmgenre