Genres are made up of similarities and differences.
An example of a similarity in e.g. horror genre, this is known to be associated and built up around reoccurring elements or features. Some reoccurring themes may be dark/low key lighting, a forest and a group of teenagers. The audience then becomes familiar with these features and associate them to the horror genre.
An example of a difference in, e.g. horror genre is the majority of movies follow a similar or the same story structure, e.g. Freitag’s Pyramid, but a few movies may change the structure of their stories a bit to have a sense of difference between these other media productions.