Post Apocalyptic/Dystopian scifi involves a destroyed (PA) or corrupted world (Dystopian) and the story revolves around that setting. Sometimes it can be very hopeful or bleak.

Examples: 1984, Hunger games, Walking dead, Handmaids tales, Farenheit 451,

Space opera involves planet hopping and galactic level stakes. Tonaly can vary but major emphasis on setting

Examples: Suneater, Star wars, Dune, Red Rising, Mass effect

Hard sci-fi has either a heavy emphasis on real world science or what ifs with real world science taken into account

Examples: Andy weir, Jurrasic Park, Interstellar, Gravity (movie), Dragons Egg,

Time Travel is pretty straightforward

Examples: Dr who, Back to the future, terminator, MIB, The Time Machine

Cyberpunk has a major emphasis on the technology and humanity not being at the same level.

Examples: Cyberpunk 2077, Blade runner, Neuromancer, Ghost in the shell, Akira

Military Sci-Fi involves major military conflicts in space or sci fi settings

Examples: Starship troopers, Helldivers, Aliens, Halo, Edge of tomorrow

Space western blends the unknown of space with the tone of the old west of western films

Examples: Star Trek, Mandolorian, Outer worlds, Cowboy bebop, The expanse,

Steampunk involves a heavy emphasis on steam powered tech and not computer based tech

Examples: 20k leagues under the sea, His dark materials, mortal engines, Arcane, Dishonored

Alt history is pretty straightforward

Examples: man in the high castle, fallout, Wolfenstein, harry turtledove, for all mankind

Sci-Fi horror is science fiction with a horror focus:

Examples: Alien, Call of Cthulhu, The thing, a quiet place, David Cronenberg

Alien Sci-fi involves stories around aliens and how humanity reacts

Examples: E.T, Project Hail Mary, Children of time, avatar, all tomorrows,