Servant

Servants are Heroic Spirits summoned by the Holy Grail. They become classified as familiars. They are pure souls invoked for the heroic spirits to be bound to, put in 7 classes prepared beforehand.

They are not bound to humans by definition, but they are to cooperate with the Master who receives a right of absolute authority in case of emergency when he performs the summoning.

They can turn into spirit form at will, under which it is possible to decrease greatly the need for prana. When Lancer invaded the Emiya mansion, he was in spirit form and turned into physical form when dropping from the ceiling into the living room.

They cannot be affected by normal interference when dematerialized, but need to materialize for battle.

Source: Fate/Side Material encyclopedia entry (p.61)