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 suit, 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.
Treemen are the most powerful and fearsome of the forest spirits that are allied with the Wood Elves.
Durthu would be the representative of the toughest and oldest treemen of all, who is constantly hunted by his need for vengeance and hatred against his old foe that wronged him, especially the dwarves, but he blamed himself the most. He also has a giant orange sword forged by Daith(a blind elven smith who supposedly not only older than Orion and Ariel themselves, but also forged Aenarion’s Dragon Armour).
In total war warhammer, tree spirits like Durthu and Treeman were speaking in some kind of alienish language in their dialogue, which is unknown if the lore specify what language they speak.