A sub-genre is a subcategory within a particular genre. Thriller is the genre we are looking at and there are many sub-genres to go with it.
Here are some examples:
Sc-fi Thriller - I Am Legend
Crime Thriller - Reservoir Dogs
Action Thriller - Deja Vu
Psychological Thriller - Memento
Supernatural Thriller - What Lies Beneath
I'm interested in making a psychological thriller because it gives me more freedom to express feelings through camera work such as rainy days depicting depression or sunny days showing happiness.