Judy and MSbelle,
The answer I give varies. It depends on the question and how it's asked.
If it's an adult and they're rude I usually just say "I have MS and it makes everything more difficult for me" and walk away. If it's a child I tell them I have Multiple Sclerosis and nothing more unless they ask. Usually a parent or adult is around and I make sure it's alright to explain. If it's another person who is disabled or also using a cane or walker I don't mind discussing my disability.
It's always a different situation so my response varies. Sometimes it's hard to be polite but I don't want to give them more to talk about!
People are generally nice and usually just curious but I do wish they'd save their "I've got a friend with" stories for someone else.