Sapwood vs. Heartwood

Sapwood is the outer layer of a tree's trunk, still actively conducting water and nutrients while the tree is alive; heartwood is the older, structural core the tree has chemically converted and effectively abandoned as living tissue. In most species heartwood is visibly darker and richer in color than the pale sapwood surrounding it -- walnut's contrast between chocolate-brown heartwood and near-white sapwood is one of the most dramatic examples.

The practical difference that matters most: heartwood is almost always significantly more decay- and insect-resistant than sapwood of the same species, because of the extractive compounds the tree deposits in it. That's why species prized for outdoor use (teak, cedar, white oak) are specifically valued for their heartwood -- the sapwood of the same tree offers little of that same protection.