By analolgy, the grey matter is like the CPU in a computer and the white matter is like the printed circuit board that connects it to the other parts of the computer.
The white matter is so-called because it contains lots of nerve fibres (neurons) that are sheathed in a white fatty insulating protein called myelin. In section, myelin is white whereas the grey matter is grey due to all the grey nuclei in the cells that make it up.
Multiple Sclerosis is predominantly a disease of the white matter in the central nervous system. About 95% of all lesions associated with multiple sclerosis occur in the white matter.
The white matter is found in the inner layer of the cortex, the optic nerves, the central and lower areas of the brain (notably the Brainstem) and surrounding the central shaft of grey matter in the spinal cord.
White Matter Links: