Genre is a way of categorising a type of media (film, TV, animation and video games) that share similar environments, character types, tropes and cliches. Inside a genre there can be various sub-genres that deal with more specific aspects that the over arcing genre doesn't delve into. These sub-genres can help in differentiating between two medias that share the same genre. Sub-Genres can also be combined, leading to a collaboration of the tropes, characters and cliches.
A good example is the Horror Genre, as the over use of horror cliches and tropes making it easy to categorise. While the main aspect of the horror genre is to scare and install fear in the viewers, there are various sub-genres that deal with specific types of horror, such as psychological horror and slasher films. The two are different in the way that they deal with horror.
Psychological horror deals with integrating fear into audiences and screwing with the mental psyche of the viewer. These films usually ending in an ambiguous way these types of films usually end with more questions then answers. Movies such as the Blair Witch Project, The Exorcist, Psycho and Black Swan are good examples of this sub genre. Slasher films on the other hand are mass killing films, filled with gore and usually involving a cast of teens/twenty years olds facing it off with a serial killer or monster. While it is classed as a horror sub genre, some Slasher movies usually begin light hearted, especially back in the 70s and 80s where the young cast of victims would usually be stereotypical teens, partying or out in the woods where the murderer is residing. Types of popular Slasher films include, the Halloween Franchise, Scream, Nightmare on Elm Street, and Friday the 13th.