Grey matter (or gray matter) is a main component of the central nervous system. It is composed of neurons (that is, the cells usually called 'brain cells') and glial cells. By contrast, the white matter is composed of long-range myelinated axon tracts (nerve fibres) and glial cells.
The colour difference arises mainly from the whiteness of myelin. In living tissue, grey matter actually has a very light grey colour with yellowish or pinkish hues, which come from capillary blood vessels and neuronal cell bodies.