The ANBU

The ANBU is actually short for 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 shoulder(Men: left shoulder. Women: right shoulder) a Ninjatō strapped on their back. They know the layout of the human body in detail, and have developed several techniques to perform their duties, such as paralyzing the target. ANBU are considered the elite, and they perform high-ranked and dangerous missions. Within the ANBU of Kirigakure,and presumably other ninja villages there exists a sub-group called Hunter-nin (literally "Chasing Ninja", English TV "Tracker Ninja"), given the specific duty of hunting down rogue ninja, and destroying their bodies, so their village's secrets won't be revealed to foreign ninjaKonoha is shown having two ANBU-like organizations, though are separate from ANBU entirely. The Konoha Military Police Force, or Military Police for short, is a unique organization in Konoha, though other villages may have similar organizations. Their symbol is the Uchiha crest because the Uchiha family founded the organization as a sign of trust with the Second Hokage. The Military Police previously consisted of mostly Uchiha clan members, though now that Itachi has eliminated all but Sasuke this is no longer the case. The Military Police are in charge of law enforcement within the village, and their jurisdiction is virtually unlimited except in matters that are under the Hokage's direct control. The Nijū Shōtai is a task force created by Tsunade, the Fifth Hokage, for the purpose of tracking down, attempting to capture, and, if necessary, eliminating Akatsuki members in the Fire Country. This force consists of twenty four-man teams, making eighty shinobi in all.
Konoha ANBU are usually under the direct command of the Hokage, but several years before the start of the series there was an ANBU Training Department known as the "Foundation". The Foundation was created and directly controlled by Danzo, who trained and conditioned children intensively from a very young age. This was done to completely eliminate emotions and enable its members to complete missions without being held back. The Foundation tends to go further in its belief that ninja must not show any emotion than most other ninja groups do, as they forbid any members to possess anything that may invoke emotion. It was implied that the department might have used methods similar to Kirigakure's graduation exam, including putting students against each other in fights to the death. Because Danzo is a war hawk and opposed the Third Hokage, the Foundation was disbanded. However, many former members are still loyal to the Foundation.