The ANBU (暗部, The ANBU? literally "Hidden Section", Viz manga "Black Ops", English TV "ANBU Black Ops") is actually short for Ansatsu Senjutsu Tokushu Butai (暗殺戦術特殊部隊, Ansatsu Senjutsu Tokushu Butai literally Special Assassination and Tactical Squad), and is an organization of assassination squads that serves directly under the village's leader. The ANBU wear porcelain animal masks in order to distinguish themselves from normal shinobi and to conceal their identities, and also have standard uniforms consisting of black and gray armor, arm guards, a signature spiral tattoo on their left shoulder.

S-rank:

S-class ninja have gained power far beyond an average Jonin and earned a legendary status. This title is merely a class rather than an actual rank; the ninja's official rank with a village will usually still be Jonin. However, it can sometimes be lower, or they can have no rank at all. The allegiances of these ninja range from being part of the village's own ninja to being wandering ninja who are still loyal to their village. Some ninja of this level tend to leave their villages behind to seek their own ambitions

Medical ninja:

Medical-nin (医療忍, iryō-nin) are ninja who specialize in medical treatment and use medical-oriented jutsu to heal others. Becoming a medic-nin requires an extreme level of intelligence and excellent chakra control, since medic-nin use chakra to heal injuries. Aside from their medical training, their combat training consists mainly of taijutsu skills, with emphasis placed on evading enemy attacks, as the death of the medic-nin could result in the death of the team, though medical techniques can also be used offensively. On dangerous missions, a medic-nin will usually be sent along with the team, to increase the survival rate of that team. Sakura Haruno in Shippuden becomes a medic-nin after her training with Tsunade.

Missing-ninja:

Missing-nin (抜け忍, Nukenin, literally "Fugitive Ninja", English TV "Rogue Ninja") are ninja who have abandoned their Hidden Village, and as such can include ninja of every rank. These ninja are considered traitors and are hunted down because of the secrets they may hold. Other Hidden Villages would be interested in purchasing this information because it could be useful in gaining an upper hand in battle or obtaining a bloodline limit unique to a village.