From Bulbapedia, the community-driven Pokémon encyclopedia.

Recalling a Pokémon is taking it out of battle. It is also referred to as switching out or retreating. Generally, trainers are allowed to recall their Pokémon at any time during the match. If a trainer chooses to send out another Pokémon in its place, the new one loses a turn. The original Pokémon is still eligible to battle later. If the trainer does not send out a replacement Pokémon, he must forfeit the match (although running from trainer battles is illegal in the games). A Pokémon is automatically recalled if it faints or is otherwise unable to battle.