Myelin is layers which have a higher proportion of lipids than other surface membranes.  Myelin is wrapped around Schwann cells.
There are two functions of myelin:1.)insulation for the axon 2.)to speed nerve conduction
Myelinated axons appear to be whiter, therefore are called white matter.  They are mostly found in the peripheral nervous system.
Unmyelinated axons appear to be gray, therefore are call gray matter.  They are mostly found in the central nervous system.