When the tree first forms a wood cell, the cell is white in colour and is wide open . It is wide open to ensure that liquids can easily be transported from the roots to the leaves. This wood is called sapwood.
As the sapwood cell ages 5-75 years) , depending on the tree species. This sapwood will begin to accumulate chemicals. These chemicals are often dark in colour (such as the black of walnut or the red of cherry) and may have a unique odour as well ( such as the sweet smell of western red cedar). Soon, this aged sapwood cell no longer can conduct liquids, at which point its only role is to support the tree. These aged cells that are usually darker in colour and have a special odour are called heartwood cells.
For most species, the white sapwood is not as pretty (to some people) as the dark heartwood. Note that a few species, such as maple are virtually all sapwood and it is the white colour that is desired, while the brown heartwood is considered a defect.