The Guardian list's the following as the main genres of film:
Action and adventure
Period and historical
Science fiction and fantasy
Whereas, popular book series 1001 Movies to See Before You Die defines films into these genres:
Art-house, Comedy, Epics, Film noir, Horror, Musicals, Romance, Social drama, Science-fiction, Thrillers, War, and Westerns.
Instantly this shows just how people everywhere struggle to truly define a film's genre these days. Living in a technical world whereby anyone can pick up a camera and make and share their own film (as we are doing), film is an ever-growing medium, and the laws that define what's what are ever changing. When you add on to the above lists all the subsidury and crossover genres you can begin to imagine just how big the world of film now is.
Wikipedia in fact states the multiple methods in which films are categorised "according to their setting, theme topic, mood, or format." Then comes the question as to who actually categorises the film, with a large part of films being about gaining a personal response from it's audience, a lot actual comes down to personal opinion.
Wikipedia then proceeds to list 65 different genres, and even then there are some missing. Clearly I cannot analyse all the genres in the world of film but I will try to cover the basics.