The Two Towers
Treebeard is the greatest of the Ents (the tree-herders who were initially awakened by the elves); his other name is Fangorn, like the giant forest in which we meet him. He is fourteen feet tall and mostly tree-like, with deep and compelling eyes. In fact, the eyes of the Ents are the most amazing things about them. Take a look at Pippin's description:
One felt as if there was an enormous well behind them, filled up with ages of memory and long, slow, steady thinking; but their surface was sparkling with the present; like sun shimmering on the outer leaves of a vast tree, or on the ripples of a very deep lake. (3.4.21)
Treebeard's eyes give Tolkien a way to show just how different the Ents are from either humans or hobbits. They come from trees, so everything about them, from their sense of time to their relation to nature, is totally different from our own. It's because of this difference between a tree's perspective and a person's perspective that Treebeard can go on to say,
I am not altogether on anybody's side, because nobody is altogether on my side, if you understand me: nobody cares for the woods as I care for them, not even Elves nowadays. (3.4.84)
Treebeard and his brother Ents are growing more and more isolated because the world is changing. It's really sad, actually: the Ents are aging and turning tree-ish in much the same way that the elves are going back into the West (about which, check out Legolas's "Character Analysis").
But while the Ents may be fading away a bit like the Elves, they also have an extra tragic reason for the disappearance of their kind. They have gone and lost the lady Ents, a.k.a. the Entwives. The Ents love the wild forests, but the Entwives prefer gardens and farmed land. So even in the good old days, they rarely actually lived together; the Ents and Entwives just met to have children (Entings) when it was convenient for both.
But during the last big war against Sauron, the beautiful gardens and fruitful orchards of the Entwives were all burned and destroyed. Now, the Entwives have disappeared entirely, to who knows where. The Ents have looked everywhere for them, but no dice.
Because the Ents no longer have any children, Treebeard tells Merry and Pippin that this war with Saruman may be the last great stand of the Ents. Again, the Ents are a lot like the elves: this fight is their last hurrah in Middle-earth.
By the way, Tolkien made some comments about the whole Ent/Entwife distinction, which we think are worth noting: "And into this [idea of the Ents] has crept a mere piece of experience, the difference of the 'male' and 'female' attitude to wild things, the difference between unpossessive love and gardening" (source, pg. 212). Frankly, we think Tolkien's "experience" that women like gardens and men like wild spaces seems like a big generalization, and more than a little strange. We see plenty of women hiking, and lots of guys like to farm. But nevertheless, this idea was floating around in Tolkien's brain as he wrote The Two Towers, so we might as well pay it some mind.
Saruman = Traitor
Treebeard is so mad at Saruman in The Two Towers because Treebeard once treated Saruman as a neighbor and friend. He shared a lot of secrets of the forest with Saruman. And instead of respecting Treebeard's knowledge, Saruman used this learning to start attacking Fangorn Forest. Not. Cool. Dude.
If it weren't for Saruman's personal betrayal of Treebeard's trust, maybe he and his fellow Ents would be less eager to flood the guy's home and knock down his fences. After all, the Ents don't like violence, and they usually try to stay above the fray when it comes to human affairs. But Saruman's useless and wasteful destruction of the Ents' beloved trees convinces them to march against Isengard with an army of thousands of trees to pull Saruman's fortress down.
These Ent-ish trees—the Huorns—are full of fury. They also go straight to Helm's Deep to destroy the orcs attacking Théoden's people. For more on this moving forest, be sure to check out our section on "Symbols, Imagery, Allegory."
One last note about the Ents: they love words. When Treebeard first meets Merry and Pippin, he is confused because he does not know how to fit the hobbits into the old songs naming the world's creatures. Their language is immensely long and slow (as you would expect from a tree language, since trees are pretty slow as a rule), but they are also familiar with all of the origins and derivations of Elvish and of Common Speech.
Tolkien has commented that, "looking back analytically I should say that the Ents are composed of philology, literature, and life. They owe their name to the eald enta geweorc ['the old creations of the giants'] of Anglo-Saxon, and their connexion with stone" (source, pg. 212).
Now, Tolkien was himself a philologist, which is someone who works with the origins of different languages. That interest in languages is one reason why Tolkien went to the trouble of inventing both High and Low Elvish for his characters to speak. So we are guessing that Tolkien put something of his own linguistic nerdiness into Treebeard and the other Ents. And why not?