The consistent conventions, themes and stereotypes around certain types of stories. Example genres include Thrillers, Detective stories, Sci-fi, Romance, Young Adult, Fantasy, Cozy Mystery, Horror and so on. The Twilight Saga successfully mixed Young Adult with Horror/Fantasy, and created a new sub-genre.
The key thing to understand is that genre’s imply their own rules and conventions. In thrillers, bullets aren’t fatal. In horror stories, it somehow makes sense to confront the axe-wielding maniac. In Romantic comedies, the leads are allowed to be willfully blind. Etc.