Sub genres is simply a genre within a genre. Sub genre movies are movies which includes the theme of more than one genre.
As I'm studying Thriller genres I have listed some thriller sub-genres, there are examples below:
Supernatural thriller an example will be Lady In The Water
Medical thriller an example will be The Experiment
Political thriller an example will be The Constant Gardener
Conspiracy thriller an example will be Edge Of Darkness
Crime thriller an example will be Hostage
Sci-fi thriller an example will be Black Swan
These are all thriller sub genres, but for me personally Sci-fi thriller inspire me due to the fact that I like the movies being released under that genre, such as Inception and Black Swan which are really great movies and they are both under Sci-fi thriller. Inception and Black Swan have made me realise that thriller doesnt necessarily have to mean that Thriller has to be scary, for example Inception isn’t scary at all, all Inception does it just plays with your mind.