Treebeard is the oldest of the Ents, an ancient Tree-like people. His name is the Westron translation of the Elvish name Fangorn.
Merry and Pippin met Treebeard as they came to Fangorn Forest after escaping from the orcs. They told all they knew and roused Treebeard. Treebeard then assembled the entmoot, a meeting of Ents and the Ents decided to attack Saruman in Isengard.