The children of the forest are a non-human species, dimunitive in size (the First Men they warred against towered over them). They had greenseers and created the faces on the weirwood trees. They worshiped the nameless and numerous old gods of the forest. They had mighty magic, and shattered the Arm of Dorne (a land bridge linking Westeros to the eastern continent) and helped build the Wall.
They vanished after the War For the Dawn 8,000 years prior to the books beginning, and the few who remained seem to have disappeared during the Andal invasion of 2,000-odd years later. The prevailing theory is that went beyond the Wall.
Aside from that, not much else is known. I have a theory that the Others have an interesting relationship with the children (perhaps the children created them to war against the First Men, didn't need them, but then unleashed them by accident a few millennia later?). We'll probably find out more about them in the later books, or indeed in ADWD.