Walk into a bookstore, and chances are you'll see books divided into sections by genre. Romance, Science Fiction/Fantasy, Mystery, and so on. It's the most common system of categorizing books, conversationally and from the data-management perspective of the book world. Genre is also incredibly limiting at times.
There are dozens upon dozens of subgenres across the genres of popular fiction (Romance, Crime, and Science Fiction/Fantasy, plus some others). Science Fiction gets sliced up into Space Opera, Mundane SF, Hard SF, Cyberpunk, Dieselpunk, etc. These subgenres can get hard to keep track of, especially since their boundaries are often porous, and even life-long fans often disagree on the borders between sub-genres, policing them inefficiently but with gusto. At times it's fun to argue classifications, try to find exactly the right place to frame a piece so that its cultural and narrative context is most clear. And narrow sub-genres can be useful for putting works into clusters for conversation, but it's also really easy to slice so thin that the discussion becomes obscure or self-serving rather than practical.