One of the most creative creatures from J.R.R. Tolkein's mind, Ents are tree like creatures that are very old and talk very slow.
In the book, Gandalf tells that the Ents are a very mighty force of Good, like how the cave trolls are a mighty force of evil.
Their main part in the Trilogy is in The Two Towers where the Ents take down Sauruman the white at Isenguard. This is done quite easily since his land is emptied and his army has gone to Helms Deep to slaughter the men of Rohan.
With a surprise attack, and the encouraging help of the two hobbits Merry and Pippin, Isenguard is taken down with ease.