what are ghost types weak against

Ghost-type Pokémon are weak to Ghost and Dark moves in the mainline Pokémon games.

Core answer

• Ghost-type takes super-effective (2×) damage from Ghost-type attacks (like Shadow Ball, Shadow Claw, Shadow Sneak).

• Ghost-type also takes super-effective (2×) damage from Dark-type attacks (like Bite, Crunch, Knock Off, Dark Pulse).

Quick extra tips

• Ghost-types are immune to Normal and Fighting moves, so avoid using those when trying to hit them; they do zero damage.

• Many Ghost Pokémon are dual-type (Ghost/Poison like Gengar, Ghost/Flying like Drifblim, Ghost/Dragon like Giratina), so their additional weaknesses (like Ground, Psychic, Rock, Fairy, Ice, etc.) depend on that second type.

If you just want the matchup chart in your head:
Hit Ghost with Ghost or Dark; never use Normal or Fighting on them.

Information gathered from public forums or data available on the internet and portrayed here.