Type chart

Every attack has a type, and every Pokémon one or two. An attack does 2× damage to a type it's strong against, ½ to one that resists it, and nothing to one that's immune. Against a Pokémon with two types, both count: 2 × 2 is 4×.

Pokémon also hit harder with moves of their own types.

Attack ↓ / Defends →	Normal	Fire	Water	Electric	Grass	Ice	Fighting	Poison	Ground	Flying	Psychic	Bug	Rock	Ghost	Dragon	Dark	Steel	Fairy
normal													½	0			½
fire		½	½		2	2						2	½		½		2
water		2	½		½				2				2		½
electric			2	½	½				0	2					½
grass		½	2		½			½	2	½		½	2		½		½
ice		½	½		2	½			2	2					2		½
fighting	2					2		½		½	½	½	2	0		2	2	½
poison					2			½	½				½	½			0	2
ground		2		2	½			2		0		½	2				2
flying				½	2		2					2	½				½
psychic							2	2			½					0	½
bug		½			2		½	½		½	2			½		2	½	½
rock		2				2	½		½	2		2					½
ghost	0										2			2		½
dragon															2		½	0
dark							½				2			2		½		½
steel		½	½	½		2							2				½	2
fairy		½					2	½							2	2	½

Each Pokémon's Pokédex page lists exactly what it is weak to, resists and is immune to.

Catch them yourself