The ANBU (暗部, Dark Side), short for Ansatsu Senjutsu Tokushu Butai (暗殺戦術特殊部隊, Special Assassination and Tactical Squad; English Version: ANBU Black Ops), 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. 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, with the exception of Root members who unless ordered to will otherwise only respond to Danzō.
Source from naruto.wikia