In Ars Magica, a Grog is a character who isn't a mage, originally some kind of mercenary or other expendable assistant. Grogs are real characters with backgrounds, statistics and personalities, but they are played by the Troupe as a whole, usually with a particular Grog played by a different player each session. Grogs are used to enhance Troupe Play by providing characters that can develop and then die.
Grog-style play is occasionally used in other games; in particular, Vampire players may play ghouls as Grogs.