"We go, we go, we go to war, to hew the stone and break the door; For bole and bough are burning now, the furnace roars - we go to war!"
- – The Ents' Marching Song, LoTR
As the name suggests, a Treeman is a towering plant-monster that resembles a tree in basic stature and appearance, but which is capable of getting up and walking around like a man. Naturally associated with dense woodlands, they tend to be averse to fire and look down on people who carelessly clear trees or start fires, not being above beating the shit out of anyone who they feel doesn't respect the forest.
Treemen are often associated with dryads, though exactly how so varies from game to game. In one setting, they may be male and female of the same species. In others, they may respect each other on a professional level, as dryads tend to be focused more on specific trees whilst treemen guard the forests as a whole.
The most famous example of treemen in fantasy comes from Tolkien's trilogy, The Lord of the Rings. Known as the Ents, they are an ancient race of humanid plants created in the dawn of the world and responsible for creating, tending and guarding the forests of Middle-Earth. Tolkien's ents are literally tree-men, being the males of the species; the females, known as the Entwives, went sailing across the sea thousands of years ago and haven't been seen since. Legend has it that this idea stems entirely from the fact that Tolkien saw a production of Macbeth and was disappointed that the prophecy of Birnam Wood coming to Dunsinane wasn't accomplished by the trees coming to life and walking there.
Naturally, Dungeons & Dragons promptly followed in Tolkien's footsteps, since the game basically began as a Lord of the Rings army game turned RPG, avoiding a lawsuit from the Tolkien estate by calling its treemen... wait for it... "Treants". Yes, it is one of the stupidest names you could imagine, but it passed in court, and it's stuck around for a good five editions so far.
For more detail, check their page.
Treemen are the most powerful and fearsome of the forest spirits allied with the Wood Elves.
They are spirits in tree form that sprouted from the power of Oak of Ages, and it happened way before the Chaos invasion. Since the Old Ones are hinted to be the ones that planted the Oak of Ages' seeds, these Treemen have a sort of extraterrestrial lineage. They're also more resilient than most mortal beings (good ol' flaming attacks work though) in addition to dealing brutal physical and magic attacks if you're unfortunate enough to piss one off up close OR at range. That's after considering the rare chance of laying eyes on one as a non-Elf.
Treemen were peaceful before Chaos invaded, even willing to communicate with outsiders like the Elves from Ulthuan. It was Treemen who taught the arriving Elves the secrets of the forest, which the Elves took to heart and taught them their language in return.
After the chaos invasion, many Treemen grew vengeful for having to acquit themselves to warfare and desperately defend the devastated portions of Athel Loren. They also became more xenophobic and are now naturally wary of outsiders, though still cautiously accepting the Elves' presence.
Speaking of Dwarfs, those beardy axe fuckers came to Athel Loren after the War of the Beard to find some Elves AND trees/tree spirits to split down the middle. Unfortunately for the Treemen, they are weak and slow to rouse during the winter and became easier targets for the Dwarfs. Much of (the then much larger) Athel Loren was destroyed before the forest spirits drew their borders back until the Elven colonists were exposed to their attacks. These Elves in particular promptly kicked their bearded asses back the way they came and eventually settled in to become the Wood Elves.
Durthu is one of the toughest and oldest Treemen of all, constantly haunted by his need for vengeance and fueled by hatred for the Dwarfs that almost killed him (but really anyone that fucks around on his lawn), although he is also weighed down by blame for himself. He also has a giant Treeman-sized fucking sword forged by Daith (a blind Elven smith who is supposedly not only older than Orion and Ariel themselves, but also forged Aenarion’s Dragon Armour).