The brain and the spinal cord form the central nervous system (CNS). Multiple sclerosis is a disease that attacks the nerve cells (neurons) in regions of the CNS called the white matter. These are regions where the neurons are sheathed in an insulating substance called myelin which is the primary target of the disease.
Here is a diagram of the brain. Move your mouse over this image to highlight different areas. By clicking on a highlighted region, you will be transferred to a page describing how multiple sclerosis can affect that region. The full-functionality of the image will take a little time to load).