Ents were created at the request of Yavanna to guard the trees, and thus were called "Shepherds of the Trees".
Ents are a very old race that appeared in Middle-earth when the Elves did. They were apparently created by Eru Ilúvatar at the behest of Yavanna, after she learned that Aulë's children, the Dwarves, were wont to fell trees. Ents were envisioned as Shepherds of the Trees, to protect the forests from Orcs, Dwarves and other perils. The Elves have tales of teaching the trees to talk, and they also taught the Ents to talk: although the Ents were sentient beings at the time, they did not know how to speak until the Elves taught them. Treebeard said of the Elves "curing the Ents of their dumbness" that it was a great gift that could not be forgotten ("Always wanted to talk to everything, the old Elves did").
Ents are tree-like creatures, having become like the trees that they shepherd. They vary in traits, from everything to height and size, colouring, and the number of fingers and toes. An individual Ent more or less resembles the specific species of tree that they typically guard. For example, Quickbeam guarded Rowan trees and thus looked very much like a Rowan (tall and slender, etc.). In the Third Age of Middle-earth, the Forest of Fangorn was apparently the only place Ents still inhabited, although the Ent-like Huorns may still have survived elsewhere, as in the Old Forest.
Almost nothing is known of the early history of the Ents — they apparently lived in and protected the large forests of Middle-earth in previous ages, and they briefly appear near the end of the First Age, attacking a band of Dwarves, apparently summoned by Beren and Lúthien. Treebeard told of a time when apparently all of Eriador was one huge forest and part of his domain, but these immense forests were cut by the Númenóreans of the Second Age, or destroyed in the calamitous War of the Elves and Sauron of the 17th century of the Second Age. Treebeard's statement is also supported by remarks Elrond Half-elven made at the Council of Elrond. Elrond said that "Time was once when a squirrel could carry a nut from tree to tree from Rivendell to the Great Sea...", further indicating that all of Eriador was once a single vast primeval forest, of which Fangorn Forest was just "the Eastern End of it" according to Treebeard.
There used to be Entwives (literally "Ent-women"), but they started to move farther away from the Ents because they liked to plant and control things, so they moved away to the region that would later become the Brown Lands across the Great River Anduin. This area was destroyed by Sauron, and the Entwives disappeared. The Ents looked for them, but have never found them. It is sung by the Ents that one day they will find each other. In the Fellowship of the Ring, Sam Gamgee says his cousin Hal saw treelike giant in the north of the Shire. When Pippin and Merry tell Treebeard about the Shire, Treebread says the entwives would like that land.
Treebeard boasted to Merry and Pippin about the strength of the Ents. He said that they were much more powerful than Trolls, which Morgoth (in the Elder Days or First Age) supposedly made as imitations of the Ents, but did not come near to their power. He compares this with how Orcs were Morgoth's imitation of Elves.
The Elvish name for Ents is Onodrim, singular Onod.
The March of the Ents
- "The Ents are going to war!"
- ― Treebeard
In The Two Towers, the second volume of Tolkien's The Lord of the Rings, the Ents—usually a very patient, deliberate people—do become angry at Saruman, whose armies are cutting down large numbers of their trees. They convene an Entmoot, a meeting of the Ents of Fangorn Forest at Derndingle.
After lengthy deliberation (though from the perspective of the Ents, this is very quick action), they march on Saruman's fortress at Isengard: the last march of the Ents. They are led by Treebeard, the oldest Ent, and accompanied by the Hobbits Meriadoc Brandybuck and Peregrin Took. They destroy Isengard in an all-out assault and trap Saruman in the tower of Orthanc. Tolkien later noted that the destruction of Isengard by the Ents was based off of personal disappointment in MacBeth, when "Birnham Wood is come to castle Dunsinane", Tolkien was less than thrilled that it amounted to men walking on stage with leaves in their hats; he decided that when he did that scene for himself, he'd do it right.
Portrayal in Adaptions
Ents in Peter Jackson's The Lord of the Rings were portrayed as, perhaps, more tree-ish than in the books (the books describe them as having smooth skin; the movie has them with skin like thick and rough bark. They have been accused of being "stupidified", though the explanation that Peter Jackson gives is that he wanted to give Merry and Pippin a more important role in the action. In the movie adaption, the Ents at the Entmoot decide that this is "not our war", despite strong protest from Merry. The scene is also shorter, cutting out Bregalad completely.
Treebeard is about to take them north to the border, when Pippin insists that they go South instead, because "the closer we are to danger, the farther we are from harm". This "does not make sense" to Treebeard, but he does as they ask, and sees the ruin and destruction that Saruman has wrought on southern Fangorn. Treebeard then calls the Ents to battle with his booming Ent-call, and they appear out of the forest as if they had been standing there waiting for it. That they do not know the borders of their own forest is another critizism. But others have accepted Jackson's reasoning as valid, because of the seemingly minor actions of Merry and Pippin throughout The Two Towers.
J.R.R. Tolkien most likely had not heard of the phenomenon, but there are actually rare trees which can "walk" by growing roots on the side in which they intend to move and killing the roots on the opposite side. 
|Treebeard (Fangorn) · Leaflock (Finglas) · Skinbark (Fladrif) · Quickbeam (Bregalad) · Beechbone · Fimbrethil (Wandlimb)|