Fangorn forest was 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 felling trees to build their mighty ships and Sauron's war on the Elves. Fangorn forest was the oldest part of Treebeard's realm, and to here the Ents retreated.
War of the RingsEdit
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 (Treebeard) 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; they also send Huorns to Helm's Deep to deal with the orcs there. Part of the reason is that Saruman's orcs had been chopping down the trees at the south and west side of the forest, which had angered the Ents.
Also in The Two Towers, Aragorn, Gimli and Legolas enter the forest to try and find Merry and Pippin, and encounter a white wizard. At first they believe it to be Saruman before realising that it was Gandalf, who they thought had been killed at Khazad-dum.
Places of Middle-earth and Arda
Forests & Mountains:
The rest of Arda: