A Faction in the context of the Total War series of games refers to one of the various competing forces in the game which may or may not be controllable by the player when starting a campaign or custom battle. During any campaign or custom battle, the player may choose one faction to play as while the remaining active ones will be controlled by the computer, or another human player in the case of multiplayer matches.
Factions are differentiated from each other in various ways in the Total War series. Each will typically have a pre-determined list of unique units which can be trained from its settlements - for example English Longbows and Scottish Highlanders. Similarly, available building choices may be limited depending on the faction you play as - for example Scotland in Medieval 2 find themselves unable to construct the necessary buildings to train gunpowder units. Thus each faction is designed to provide a unique challenge to the player when either played as or met on the battlefield.