In J. R. R. Tolkien's Middle-earth, Fangorn forest is the habitat of the Ents. It is named after the oldest Ent, Fangorn (Treebeard) (or perhaps vice versa).
Spoiler warning: Plot or ending details follow.
Meriadoc Brandybuck and Peregrin Took pass this forest in the second volume of The Lord of the Rings, The Two Towers. There they meet Fangorn and persuade him of the danger that Saruman poses to the Ents and their forest. Following the Entmoot the rest of the Ents finally agree to march against Isengard taking Merry and Pippin with them and send Huorns to Helm's Deep to deal with the Orcs there.
Fangorn forest is actually just the easternmost survivor of the immense forest that spanned all of Eriador and Calenardhon in the First Age and early Second Age, but which was destroyed by the Númenóreans and Sauron. Fangorn forest was the oldest part of Treebeard's realm, and here the Ents retreated.
FangornForest was located at the southeastern end of the Misty Mountains near the Gap of Rohan.
To the east and south of Fangorn was the land of Rohan.
Later, in FangornForest, Treebeard told Merry and Pippin that the Old Forest was once part of the great wood that stretched from Fangorn to the Mountains of Lune in the northwest.
Tolkien’s fictional universe of Middle-earth, the Old Forest is a small forested area which lies east of the Shire.
The Old Forest is one of the few survivors of the primordial forests which covered most of Eriador before the Second Age, and it once was but the northern edge of one immense forest which reached all the way to Fangornforest.
At the south-eastern edge of the forest, on the bank of the river Withywindle, stood the house of Tom Bombadil, who rescued the Hobbits when they were trapped by a tree Tom called Old Man Willow.