Weirwood is a species of deciduous tree found all over Westeros; it has white bark with five-pointed, blood-red leaves and sap. The heart trees are weirwood trees with faces carved on their bark. The Children of the Forest are thought to have carved the faces that decorated the heart trees.
In the books
Weirwood trees live an extremely long time, and even when they die they do not rot. Objects made of weirwood are as lasting as stone. This is how the faces in the weirwood trees could actually be carved by the Children of the Forest, even though they disappeared six thousand years ago. Weirwood trees like the ones in the Winterfell godswood are thousands of years old. Even dead weirwood trees like the one at House Blackwood's seat of Raventree Hall can remain freestanding for thousands of years even after they die.