Anbu

The is actually short for. The name is a form of contraction similar to English acronyms like SWAT, where the two kanji used to abbreviate the group's full name constitute an actual word on their own.

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, 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 Ninjato, 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 Foundation ("Ne." or "root"). The Foundation was created and led by Danzo, who sends its members on missions that he believes will benefit Konoha. Foundation members are trained to abandon all forms of emotion in order to follow Danzo's orders unquestioningly. Foundation members also have a seal located on their tongue. This seal prevents a member from speaking about Danzo and the Foundation. This is to ensure that, if a member ever leaves or gets captured, information regarding the Foundation will not be leaked. During Pain's invasion of Konoha, Danzo instructed the Foundation not to assist in the defense of the village.

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.