From SRD: A treant’s leaves are deep green in the spring and summer. In the fall and winter the leaves change to yellow, orange, or red, but they rarely fall out. A treant’s legs fit together when closed to look like the trunk of a tree, and a motionless treant is nearly indistinguishable from a tree.
A treant is about 30 feet tall, with a "trunk" about 2 feet in diameter. It weighs about 4,500 pounds.
Treants speak their own language, plus Common and Sylvan. Most also can manage a smattering of just about all other humanoid tongues—at least enough to say "Get away from my trees!"