Whats the difference between Heartwood and Sapwood?

Sapwood and Heartwood Walnut

The sapwood is usually less dense than the heartwood and will not change or darken significantly over time.  The heartwood is less susceptible to rot, decay, and insect attacks, though this doesn't really apply to properly treated lumber or indoor furniture. 

Outer Sapwood Inner Heartwood

Heartwood and Sapwood are part of the natural growth of a tree and makeup all hardwood lumber. The sapwood is the wood closest to the bark of a tree.  It is often a lighter tone than the heartwood, which is the wood closest to the center of the tree.  On a living tree, the sapwood carries the water and nutrients up from the ground.  The heartwood is considered somewhat dead but provides strength and support. Heartwood and sapwood vary in color, density, and width by species.

In Furniture:

Above, the sapwood can be seen as the lighter streaks in the center of the Walnut cabinet and the heartwood as the darker brown.  It can be used aesthetically to add contrast to an otherwise uniform tone.  Its mostly aesthetic taste as both are suitable for building furniture and are equally strong, though sapwood could dent easier in some species. Remember wood lumber varies greatly,  If you like a  certain coloration or tone, let your craftsperson know, though sometimes this may cost more if they have to cut out sapwood or pick special boards.