Ms is usually a divorced woman or one who is older but unmarried. Miss.is often associated with young women and many older women do not like to be called Miss because it implies a young naive girl. Ms can also be used when addressing someone who you are not sure of their marital status
It's a new, politically correct abbreviation used by those women who consider that they shouldn't be categorised by their marital status. I once asked a feminist friend (yes, I do have them) whether it should be pronounced MSSS or MIZZ and she replied, "Mizz. It rhymes with mind your own bizzness."