From A Wiki of Ice and Fire
The godswood is usually a small wooded area enclosed by the castle walls throughout the Seven Kingdoms. It is primarily used as a place of worship by those who carry on the traditions of the First Men and pray to the old gods. However in those places that believe in the Faith of the Seven, it is little more than a private wooded area of solitude. When the First Men took up the Old Faith, they created godswoods, groves within their castles and villages where a single Weirwood known as a heart-tree, was planted so the gods could be worshiped. In southern castles such as the Red Keep the heart-tree is not a weirwood but a common tree such as an oak.
The Godswood at Winterfell is a dark, primal place of 3 acres of old forest untouched for 10,000 years. It smells of moist earth and decay, consisting of stubborn sentential trees armored in gray-green leaves of might oaks crowded close together with a dense canopy overhead. In the center is a small dark cold pool with an ancient Weirwood with bark white as bone, and dark red leaves with a long and melancholy face carved in the bark, its deep-cut eyes red with dried sap. Eddard Stark calls the tree the Heart tree. A thousand years of humus covers the earth.