I'm not that great at explaining what MS is, I just know what I've experienced and what others tell me. But, basically, Multiple Sclerosis is a disease of the nervous system that is unpredictable and variable. There are multiple lesions on the brain and/or spinal cord, which may or may not cause noticeable disability. MS is different in each person with MS, and it also varies at different times in each individual. Symptoms range from vision problems to bladder/bowel problems to numbness to the loss of use of limbs.

Here's one definition that I found:

• a disease of unknown origin (thought to be due in part to a dysfunction of the immune system) that affects the central nervous system and results in the progressive loss of certain body functions and physical abilities

So, there it is, in more concise terms (I tend to ramble, I know!)

You can find helpful information at this site:

The National Multiple Sclerosis Society

and many others listed on my two links pages.