Anbu

The, short for , 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.

There are apparently no true ranks within the ANBU, as is the case with the regular forces. Team leadership and hierarchy seem to be based on merit and experience. The leaders of the teams are called, a position held in high regards.

Uniform
The ANBU wear porcelain animal masks in order to distinguish themselves from normal shinobi and to conceal their identities. They also have standard uniforms consisting of black and gray armor, metal arm guards, and a signature spiral tattoo on their left shoulder (female ANBU members have the tattoo located on their right shoulder to determine gender between the ANBU members). Some ANBU wear black cloaks over their standard uniforms, and squad leaders may wear white cloaks. All ANBU carry katana, which are usually strapped to their backs.

Purpose
ANBU are under direct control of their Kage. They protect the village from exceptional threats, conduct high-risk missions into enemy countries, 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. If an ANBU is mortally wounded in battle, they are required to destroy their bodies to prevent any information from getting into enemy hands.

In Konohagakure, there exists a special branch of the ANBU known as the Root. It was created and led by Danzō, who sends its members on 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. Root members also have a seal located on their tongue. This seal prevents a member from speaking about Danzō and Root. This was to ensure that, if a member ever left or got captured, information regarding Root would not be leaked. During Pain's invasion of Konoha, Danzō instructed Root not to assist in the defense of the village.

Kirigakure also has their own special branch of ANBU, known as the Hunter-nin. They solely hunt down missing-nins and kill them, then retrieve the head as proof, while disposing the rest of the corpse on the spot to not leave behind any information for the enemies. Due to their duties, they are codenamed as the "Undertaker Squad".

ANBU have a detailed knowledge of the human body, and can use such jutsu as Temporary Paralysis Technique and Time Reversal Technique to take advantage of this.

Speculation
In Episode 157 of Naruto Shippuden, an ANBU reported to the Godaime Hokage Tsunade as a member of the Analysis Corps. Whether this is another sub-division of the ANBU like the Root Foundation or simply a mistake in the animation remains to be seen.