Ancillaries are used in both the Rome: Total War and Medieval II: Total War games to control character abilities. They are meant to represent either a characters retinue, or an office he has been appointed to. If a character gains an ancillary there will be a small picture representing the ancillary in his character information screen.
Some ancillaries can be transferred between generals, to do this place both generals in the same army and drag the ancillary picture from one general's information screen onto the other general's portrait. In M2TW not all ancillaries are moveable, it is also not possible to give a general more than one of the same ancillary, or to combine certain ancillaries if they are designed to be mutually exclusive.
Ancillaries are assigned according to triggers that fire under certain circumstances, eg. when a general wins a battle, or stays in a settlement that has a certain building, or when a spy successfully completes a mission.