I made a hub about this. You can check it out in my profile.
MS is basically an auto immune disease where the body attacks the sheath (myelin) that covers the nerves. This then means that the nerve can't carry the electrical signals across the body properly and then you start to get symptoms like pain, tremors, loss of sight etc. The symptoms are so varied because it depends on which nerves are attacked in the body. Sometimes it can take 10 years for a diagnosis because a lot of types of MS go into remission so the patient thinks they are getting better until they get another flare up.
Hope that helps.