"A large Man-like, almost Troll-like, figure, at least fourteen foot high, very sturdy, with a tall head, and hardly any neck. Whether it was clad in stuff like green and grey bark, or whether that was its hide, was difficult to say. At any rate the arms, at a short distance from the trunk, were not wrinkled, but covered with a brown smooth skin. The large feet had seven toes each. The lower part of the long face was covered with a sweeping grey beard, bushy, almost twiggy at the roots, thin and mossy at the ends. But at the moment the hobbits noted little but the eyes. These deep eyes were now surveying them, slow and solemn, but very penetrating."
They're difficult to define. I suppose the best way to describe them is sort of a half-way point between a tree and a man. Their skin is described as tough and bark-like, but as far as I know they didn't have literal bark for flesh. They also took on the attributes of the trees that they personally shepherded, so some would have smoother skin while others were more coarse. Their hair and beards were made up of moss and leaves, and Treebeard at least had deep green eyes.
Peter Jackson's Ents skew a bit too close to being trees for my tastes (they look more like Ents changing into Huorns), but they're still beautifully designed and perfectly valid interpretations of Tolkien's texts.
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