The user attacks everything around it by swamping its surroundings with a giant wave.
Surf does damage to the target. If used underwater, Surf's power will be doubled. In Generation III, during a Double Battle, Surf targets both foes. As of Generation IV, during a Double Battle, Surf targets all Pokémon around the user. In a Triple Battle, Surf only targets Pokémon that are adjacent to the user.
Outside of battle
Surf can be used to travel across ponds, lakes, seas, etc.