Genre is what splits up most forms of media into categories to which it is easier for anyone to find a specific film, game or even story. For example, a films genre can be Horror, Action, Sci-fi and many more. Not only this, but games can also be split into genres, such as FPS (First person shooters), Survival, Stealth etc. And finally story and be split into different genres, again with it being a Horror story, Adventure, Fantasy and many more.
What splits these genres apart and what makes them different to each other is what that specific genre contains and does to the audience. For example, Comedy(Comedic) story will have points throughout the story to which it will make the audience laugh or smile due to scene being funny. Another would be Romance, where the story will include a romantic couple and tells the story of how loved up the couple are or the opposite, why they broke up etc.
Sub-genres
Much like the main concept of a genre, a subgenre is a genre broken down into more genres. For example Science Fiction (Sci-fi) can be broken down into Atomic Age, Cyber Punk and End of the World. It narrows the genre down into specific genres within the main genre. These Sub-genres could also be a indication as to how it defines the main genre, for example a film about the end of the world as this is already a sub-genre.
References
1) Print screen of my genre table
1) Print screen of my genre table