The Old Forest is a small forested area which lies east of the Shire. It 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 Fangorn Forest.
It is bordered in the east by the Barrow-downs, and in the west by The Hedge, a large wall which the Hobbits of Buckland built after they cut the forest to make room for their new homes. When the trees grew too close to the hedge, hobbits cut down the trees nearest and created a clearing by a bonfire. Ever since then, the trees were more hostile.
The Hobbits believed the trees of the Old Forest were in some manner 'awake', and were hostile. They swayed when there was no wind, whispered at night, and misled travellers deeper into the forest. This was a Hobbit legend well-based on fact. The trees did indeed carry a malice and a hatred, but this hatred not connected to the evil stemming from the East and Mordor--the hobbits mostly referred to the forest as queer and dangerous rather than evil.
The forest had been angered by the years tree-felling and destruction that they had witnessed, and the trees would lash out at travellers, whether they meant good or ill. Frodo, Merry, Pippin and Sam were waylaid by a falling branch as they passed through, and the Hobbits had the sense that they were being watched by the forest itself. The paths in the forest, which seemed at first to be true, would angle downward into the valley of the Withywindle.
The Withywindle, which lay roughly in the center of the Old Forest, was the root of the queerness, malice, and the general animosity that the trees projected. Old Man Willow lived within the valley and seemed to have some measure of control over the forest, inspiring much of the anger that permeated the limbs of the trees.
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 Old Man Willow. Tom Bombadil seemed to represent the good side of the forest, and had some power over Old Man Willow. The two contrasting wills, of Bombadil and the Willow, brought about the aura of mystery and queerness that the Old Forest seemed to inspire.
It is probable that some trees of the Old Forest were Huorns.