What is ANBU root?

Answer 1

this is what wikipedia has to say.

"onoha 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 "Root" (根, Ne?)(Viz Manga "The Foundation"). Root 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. Root 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 (though Sai somehow has a book that he claims was for his brother). 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, Root was disbanded. However, many former members are still loyal to Root."