It’s important to understand sub-genre so you can better focus the development of your scripts. You need to know what you’re writing when you write it; you need to know what you’re selling if you’re going to sell it.
Action-adventure. An action movie that includes an element of travel, exploration, mystery and problem-solving. The Indiana Jones franchise is gold standard, but more recently we can include the National Treasure films.
Action-thriller. An action movie that is focused on grounded, “realistic” action, and isn’t working to deliver consistent laughs. The Bourne movies are action-thrillers, as are any Jack Ryan or Jack Reacher film/TV titles.
Action-comedy. An action-comedy is built around action set-pieces, while also consistently working to deliver laughs like a comedy. The Bad Boys and Rush Hour films are perfect examples of action-comedy.
Dramedy. A drama that is often funny, and/or a comedy that isn’t afraid to engage in significant character work or veer off into serious dramatic business at the expense of laughs. The films of Wes Anderson and the Duplass brothers might often be characterized as dramedy.
Romantic comedy. “Romcom” for short; a comedy with a romance as its A-story. When Harry Met Sally is a classic example, with Crazy Rich Asians being a more recent hit.
Horror-comedy. A horror movie that is also working to deliver laughs, e.g. An American Werewolf in London or Cabin in the Woods. Note that a comedy that includes horror elements (zombies, vampires, etc.) may often be seen as a horror-comedy, as will a film that is built on an inherently comedic premise, no matter how serious it’s trying to be. For example, I once had a writer get angry with me for referring to his script about a giant killer hamster as a “horror-comedy.” Well…
Action-horror. Action movies that include horror elements. Aliens is the perfect example of an action-horror movie.
Supernatural thriller. This is basically the higher-brow version of a horror movie. It’s what separates The Sixth Sense from, say, A Nightmare on Elm Street. Though both are working to scare the audience, a supernatural thriller is typically seen as a classier gig, and has a higher likelihood of attracting higher-name talent.
Sci-fi. Though sci-fi is often marketed as its own standalone genre, when developing sci-fi projects it’s better to think of science fiction as a “super genre” that is bolted on top of one of another genre or sub-genre.
For example, Aliens is sci-fi action-horror, as is Predator. Sci-fi horror: Event Horizon and Splice. Sci-fi action-adventure: Star Trek and Star Wars. Sci-fi comedy: Idiocracy and Sleeper. Sci-fi action-comedy: Galaxy Quest and Men in Black. Sci-fi romcom: Passengers. Sci-fi action-thriller: The Terminator and (on the TV side) The Expanse. Sci-fi horror-comedy: Critters and Slither. Sci-fi drama: Solaris.
Western. Western is exactly like science fiction in that they are marketed as their own genre, but when it comes to developing the scripts and films it is better to understand that “Western” is a super-genre bolted onto a genre or sub-genre.