The ANBU is a special squad of ninja specifically created for assassinations, stealth missions and dealing with exceptionally dangerous threats too dangerous for regular ninja
ANBU stands for ANsatsu Senjutsu Tokushu BUtai or Elite Assassination and Tactical Force. ANBU is composed of handpicked shinobi from the village and are in a different league from normal forces. ANBU work directly under the Hokage. Everything pertaining to them and their special missions is treated with absolute secrecy. They wear animal masks to hide their faces, and any information they obtain goes through countless security checks and procedures. This information is showed in omake of episode #50 of Naruto Shippuden.
ANBU is a special squad of highly trained ninja under the complete and direct control of the village's head or even more so the village Kage. They are specifically there to perform special high-level missions to dangerous for regular shinobi (ninja), torture and even assassination. The shinobi in the ANBU are directly chosen by the village head for their individual exceptional skill. There are no ranks in the ANBU and team leaders are chosen based completely on experience and merit.
They are designed specifically for dealing with extremely dangerous ninja, highly dangerous stealth missions into enemy countries and performing exceptionally high-risk and level missions. They also carry out various assassinations, tracking, missions that require specially trained ninja and they also provide surveillance due to their ridiculously high stealth skill. Certain ANBU may serve as interrogators or they manipulate the minds of their enemy ninja to gain information that may valuable to the village.
An interesting fact about the ANBU is that if an ANBU agent is fatally wounded in battle they are required to destroy their bodies leaving no trace behind in order to prevent any information from getting in the possession of the enemy.
There is a special branch of the ANBU in the Konohagakure (The Leaf Village), this special branch is known as "The Foundation" or "Root", this special brand was created by Danzo (who is currently serving as the current Hokage). This special branch sends their ANBU agents on missions believed to be beneficial to the village. Members of The Foundation are forcibly trained to repress or otherwise abandon all forms of emotion in order to follow Danzo's order with question and to further enforce this rule Foundation members have seals on their tongue to prevent a member of The Foundation to speak about Danzo and The Foundation to the public, enemies or anyone not associated with The Foundation. This also ensures that any member that may be captured or ever leaves The Foundation, information about The Foundation will not be revealed.
During the events of Pain's invasion of Konohagakure, Dnazo strictly ordered The Foundation not to assist in the defense of the village. The ANBU apparently have a very detailed knowledge of the human physiology and biology and are able to use jutsu such as the Time Reversal Technique and the Temporary Paralysis Technique to their advantage.
The ANBU have specifically designed uniform for them. They wear porcelain animal masks so that they may be easily distinguished from regular shinobi and also to conceal their identities from the general public as well as enemy ninja. Their standar uniform consists of gray or black armor, a signature spiral tattoo on their left shoulder and arms guards. In order to determine gender between the ANBU, female members have the tattoo located on their right shoulder. Some ANBU may wear black cloaks over their standard uniform and squad leaders may can sometimes be seen wearing white cloaks. All ANBU carry Ninjato which are usually strapped to their backs.
The ANBU is a very tactical squad always having a strategy and plan ready for highly dangerous missions.