A film style is a recognizable group of conventions used by filmmakers to add visual appeal, meaning, or depth to their work. It can encompass every aspect of film; dialogue, cinematography, attitude (i.e., seriousness or lack thereof).
Film style is distinct from film genre, which defines what a film is about - - Western films are about the American West, love stories are about love, and so on. Although some styles are strongly associated with certain genres, a style can be applied to any genre - - Barbarella - is a surrealistic science fiction film, for example.

Obviously each filmmaker develops their own film style, but certain styles that transcend individuals include:

film noir
surrealism
musical film