Ents, at first look, are big trees. When the Ents start to move around they become a more human like creature. Do they breath and eat or do they use photosynthesis? Are they made of real wood or is it a skin-like covering? many more questions could be asked.
So, the big question is, are Ents more like trees or humans?
It is nothing conclusive that they are sentient trees, but there is plenty of circumstantial evidence that they are:
High up, almost level with the tops of forest-trees, there was a shelf under a cliff. Nothing grew there but a few grasses and weeds at its edge, and one old stump of a tree with only two bent branches left: it looked almost like the figure of some gnarled old man, standing there, blinking in the morning-light.
The drink was like water, indeed very like the taste of the draughts they had drunk from the Entwash near, the borders of the forest, and yet there was some scent or savour in it which they could not describe: it was faint, but it reminded them of the smell of a distant wood borne from afar by a cool breeze at night.
...
He poured them out two full bowls from a stone jar; but from a different jar. The taste was not the same as it had been the night before: it was earthier and richer, more sustaining and food-like, so to speak.
Some of us are still true Ents, and lively enough in our fashion, but many are growing sleepy, going tree-ish, as you might say.
...
'Some of my kin look just like trees now, and need something great to rouse them; and they speak only in whispers.
One of them, Beechbone I think he was called, a very tall handsome Ent, got caught in a spray of some liquid fire and burned like a torch: a horrible sight.
It belonged to 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.
However they definitely do breathe:
For a moment Treebeard stood under the rain of the falling spring, and took a deep breath; then he laughed, and passed inside.
Finally, Tolkien wrote the march of the Ents as a counterpoint to Shakespeare's "coming of Great Birnam wood", imaginging it as if it were done by real trees. This is more circumstantial evidence that Ents are meant to be trees in and of themselves:
Their part in the story is due, I think, to my bitter disappointment and disgust from schooldays with the shabby use made in Shakespeare of the coming of 'Great Birnam wood to high Dunsinane hill': I longed to devise a setting in which the trees might really march to war. (Letter 163)
I have always read it that they are like sentient trees. Their life and processes are substantially tree-like, but they have sentience, language, and they can move. I believe this is all that is human-like about them.
If they fail to use their facilities, they tend to revert to being trees, or at least, as close as makes no difference.
However there precise physiology is not detailed. They probably use photosynthesis, enhanced by the Ent-draught. They are probably made of real wood, not just a skin. They are trees plus, I think, rather than a half of each - if anything they would be elven rather than human. But in truth, as they do not appear in the creation stories, I guess they have their own distinct origin.
The word "Ent" was taken from the Anglo-Saxon (Old English) word ent, meaning "giant". Tolkien borrowed the word from the Anglo-Saxon phrases...
In a sense, Ents are giant creatures resembling humans and trees in structure, etc.
This giant or large form they hold, is common to both trees and human-like creatures (trolls, orcs, giants). All these creatures are some form of a halfling, half one part (tree, lizard, spirit, etc), half human.
In another sense, Ents have been known to represent an entire forest (Treebeard or Fangorn).
Ents eat, or rather drink a specific water beverage.
Ent-draught was an extremely invigorating drink of the Ents, brewed from the waters of the mountain springs on Methedras.
So, Ents are more than just trees who resemble humans in form, are known to speak, walk, drink, etc. They share a half of human nature/resemblance, a half of tree nature/resemblance, and a half of their own kind Ent-kind.
Ents are concieved as being more tree-like, as is confirmed by Yavanna's recollection of the Music of the Ainur in the Silmarillion chapter 2:
'Yet it was in the Song,' said Yavanna. 'For while thou wert in the heavens and with Ulmo built the clouds and poured out the rains, I lifted up the branches of great trees to receive them, and some sang to Iluvatar amid the wind and the rain.'
Tolkien's primary interests, as is well-known, were history and language, so we don't see him going into much detail on the biological functions of his invented species.
I have always thought that Ents are fairly man-like in appearance, rather than tree-like; they are called "tree-ish" if they don't move, due to their size and the fact that they wear tree-bark; but their figure is "man-like, almost troll-like." Ents are not trees, any more than shepherds are sheep, i.e. Ents were created by Eru to guard trees against Dwarves.
But like Treebeard said, shepherds will tend to look like sheep by wearing sheep-skins and leather headbands etc, and so it stands to reason that Ents make clothing out of bark, and became more aged by the sun (as Treebeard said happened with the Entwives) until they looked rather like trees-- as anyone will if they wear treebark-camo and put their arms up.
But as usual, artists are prone to exaggeration and mis-readinjg of context, with little interest in researching authorial intent; and so they almost universally portrayed Ents exactly Disney talking trees-- just like they always portray balrogs as traditional winged hellbeasts just because Tolkien referred to them as "demons;" meanwhile Tolkien's world is completely different from Judeo-Christian lore, with Balrogs being basically like huge men that can emanate both fire and shadow from their bodies, and carry whips of flame.
And so going by artistic impressions, we might as well view Tolkien's Elves as the type working for Santa... or Keebler.
By real-world standards, if it has complex thoughts, moves under its own power, produces sounds via vocalization - especially speech - and has a full array of emotions, it must be an animal, and since humans are animals, Ents are more like real-world humans.
But in Tolkien's world, they are clearly more like trees than they are like people. Ents can become "treeish" and trees can become "Entish", for one thing. An Ent that has become thoroughly treeish is basically a normal tree - it doesn't walk or speak as often, if ever. And unlike trees in our world, at least some trees in Tolkien's world can do things like move around, feel emotions (they are frequently described as hostile, resenting things that go about on legs or wings), move around a bit (like the Huorns, who seem to be trees that have become entish, or Old Man Willow, who attacked the hobbits and may have been a Huorn). Treebeard says that he knew many of the trees - and he calls them trees - that were cut down by Saruman's Orcs, and he refers to them as "friends" who "had voices of their own".
What does all of this mean? It means that, whereas in our world the difference between trees and animals is pretty obvious, in Tolkien's world, the boundary is much harder to define, if it exists at all. Ents are clearly "more than trees", but the difference is one of degree rather than essence. In other words, Ents are more sentient than trees, and a very treeish tree is not an Ent, but as someone mentioned earlier, an Ent is basically a tree PLUS something else. The difference between trees and animals in Tolkien's world is very vague; the difference between trees and Ents is almost nonexistent.
The most we can say with certainty is that Ents are related to trees, and resemble trees to a great extent: they have bark (or something like it); they seem to drink but not eat; they have branches and root-like toes; they can become "treeish" and essentially turn into trees. Still, they act a lot like people and other humanoid beings (Elves, Wizards, etc): they walk around; they talk; they think; they have legs, a face, arms, heads, hands, feet, and even toes; they form relationships with trees, other Ents, humanoids, and possibly non-humanoid animals; they have complex emotions; they have "wives"; and so on.
But most of the things they have in common with humans are not unique to humans, or even to animals, in Tolkien's world. At least since the Elves "woke" some of the trees and taught them to speak, many normal trees have been able to think, move, speak, and feel emotions, perhaps not as fully as Ents or animals can, but to some degree at least.
So the answer is a complicated one: By our standards, Ents are more like people than trees, because they can do things trees in our world can't. But our standards don't apply to Middle-earth, and by Middle-earth standards, Ents are more like trees than anything else. They aren't trees, or at least they didn't start out as trees, but they can become something that is essentially indistinguishable from a tree.
In a world that follows the laws of magic rather than science, and where mythology has not yet been replaced by history, this question probably doesn't make much sense. An Ent is an Ent, a tree is a tree, a person is a person, and the boundaries between them are vague at best, perhaps even nonexistent. No one in Middle-earth seems to think about these things much - like the relationship between humans and hobbits, for example. Hobbits see humans ("Men" or "Big People") as a separate species, but there is reason to believe that hobbits might be a type of human. This is an interesting question to us, but apparently not to hobbits and humans in Middle-earth.
