Short answers, pulled from the story.
Ents are giant humanoid creatures in J. R. R. Tolkien's Middle-earth who closely resemble trees. They serve as ancient shepherds of the forest and allies of the free peoples during the War of the Ring. Their leader is Treebeard of Fangorn forest, described as the oldest living creature that walks in Middle-earth.
The word Ent comes from the Old English ent or eoten, meaning giant. Tolkien borrowed it from the Anglo-Saxon poems The Ruin and Maxims II, where the phrase orðanc enta geƿeorc describes Roman ruins as the cunning work of giants.
Tolkien stated in a letter that he created Ents because of his bitter disappointment with Shakespeare's Macbeth, where the prophecy of Birnam Wood coming to Dunsinane was fulfilled by men carrying branches rather than trees actually marching. He wanted a setting where trees would truly go to war. Scholars also link the Ents to his concern about damage being done to the English countryside in the 20th century.
The Entwives moved east to the region that became the Brown Lands because they preferred cultivating and controlling plants, unlike the Ents who favored wild forests. Their gardens were destroyed by Sauron, and the Entwives disappeared entirely. Tolkien wrote in letter number 144 that he believed the Entwives were destroyed during the War of the Last Alliance, though some may have fled east or been enslaved.
Roughly 50 Ents, led by Treebeard and accompanied by hobbits Meriadoc Brandybuck and Peregrin Took, marched on Saruman's fortress at Isengard, along with an army of Huorns. They tore down the walls of Isengard and destroyed the fortress, leaving Saruman trapped in the tower of Orthanc.
Ents appeared in the original Dungeons and Dragons white box set published in 1974, described as tree-like creatures able to command trees. In 1975, Elan Merchandising, which held the Tolkien estate game licence, issued a cease-and-desist over the name, so the creatures were renamed treants.