Short answers, pulled from the story.
The Vala Aulë and his wife Yavanna created the Ents as the Shepherds of the Trees. Yavanna appealed to Manwë to protect the forests after Aulë created the Dwarves, leading to the Ents being brought into existence. They were designed to ensure that the trees were not forgotten in the grand design of the world.
The Ents learned to speak when the Elves taught them language and cured them of their dumbness. This event occurred when much of Eriador was a continuous forest so vast that a squirrel could travel from what is now the Shire to Dunland without ever touching the ground. The Elves granted them the gift of language to allow them to communicate with other beings.
The Entwives moved to what became the Brown Lands across the Great River Anduin. They interacted with Men and taught them the art of agriculture in this region before their gardens were destroyed by Sauron. Their disappearance left the male Ents to wander in grief and never fully resolve their separation.
The Ents numbered about fifty during the Last March of the Ents. They were led by Treebeard and accompanied by the hobbits Meriadoc Brandybuck and Peregrin Took. They also brought an army of Huorns to destroy Saruman's fortress at Isengard.
The word Ent is derived from the Old English ent or eoten meaning giant. Tolkien borrowed the word from a phrase in the Anglo-Saxon poems The Ruin and Maxims II which translates to cunning work of giants. In Sindarin the word for Ent is Onod and the plural Enyd refers to the Ents as a race.
Ents appeared in the earliest edition of the roleplaying game Dungeons and Dragons in the 1974 white box set. In 1975 Elan Merchandising issued a cease-and-desist order regarding the use of the word ent so the creatures were renamed treants. The TV series The Lord of the Rings: The Rings of Power features Ents in the season two episode Eldest.