GENRES 

A category of artistic composition, as in music or literature, characterized by similarities in form, style, or subject matter.

SUB-GENRES 

Sub-Genre is a sub category within a particular Genre. Similar to Genre there are different types of categories within the group, an example of Sub-Genres in the film category; Romantic Comedy (Rom-Com), Action Thriller, Action Adventure, Musical Horror etc. 

Sub-Genres are effective as it could make films more effective as they cross Genres together and to also extend the ending of the film as they 'entertain' two different Genres.

Psychological Thriller

Romance Thriller