The ANBU , short for Ansatsu Senjutsu Tokushu Butai (Literally meaning “Special Assassination and Tactical Squad”, take orders directly from the Kage, performing special high-level missions, such as assassinations and torture.
The ANBU usually work in teams formed to the requirements of the mission, ensuring maximum success. The shinobi in the ANBU are hand-picked by the Kage, chosen for their individual capabilities and special skills.
Age, gender, background, or previous rank bear no weight in this decision. Members can also leave the organisation, as Kakashi did for unknown reasons to return to the regular forces. They wear their mask even in their own village, for their identity as an ANBU agent is to be known only by the Kage and village elders.