The Anbu (暗部, English TV: Anbu Black Ops, Literally meaning: Dark Side), short for Ansatsu Senjutsu Tokushu Butai (暗殺戦術特殊部隊, Literally meaning: Special Assassination and Tactical Squad), are covert operatives dispatched by their Kage.
Anbu are recruited from their village’s standard shinobi forces, hand-picked by their Kage for their individual capabilities and special skills. Age, background, gender, or previous rank bear no significance in this decision. While on the job, Anbu wear masks and use code names to conceal their identity, leaving only their Kage informed. They typically work in teams, formed on the mission’s requirements. There are apparently no true ranks within the Anbu; team leadership and hierarchy seem to be based on merit and experience. The leaders of the teams are called squad leaders (分隊長, Buntaichō), a position held in high regard.
Anbu protect the village from exceptional threats, conduct high-risk missions into enemy territory, and deal with extremely strong ninja. They are also responsible for carrying out assassinations, tracking, surveillance, and missions requiring specially trained ninja. Some Anbu serve as interrogators that probe the minds of enemy ninja to learn information valuable to the village. Because Anbu are so vital to the village’s operations, they cannot be arrested by regular police forces without a warrant. While many choose to serve in the Anbu for their entire careers, they can return to the standard forces if they choose to, as in the case of Kakashi Hatake.
Anbu training involves detailed studying of the human body. As such most Anbu can use the Temporary Paralysis Technique and, in the anime, the Time Reversal Technique. In Kirigakure, there is a special branch of Anbu known as hunter-nin: hunter-nin track down and kill missing-nin from Kiri, retrieving the head as proof and disposing of the rest of the body so as to not leave behind any information for their enemies. Due to the often sensitive nature of their missions and the, at times, unique abilities they possess, Anbu are expected to destroy their bodies if they are too badly injured on a mission so that they can’t be captured.
In Konohagakure, there exists a special branch of the Anbu known as Root. It was created and led by Danzō Shimura, who sends its members on more underhanded missions that he believes will benefit Konoha. Root members are trained to abandon all forms of emotion in order to follow Danzō’s orders unquestioningly, even at the consequence of the rest of the village. Root members have a seal located on the back of their tongues that prevents them from speaking about Danzō or Root.
While Anbu are usually in squads and take orders from their captains, some elite Anbu have been handpicked by their Kage to serve directly under them. As such, any and all orders they receive come from their Kage and no one else.