To answer this question I'll need to include the abbreviation Mrs as well as Ms and Miss. Both Miss and Mrs are abbreviations of the term mistress. Miss was first used in 1645 to describe a 'kept mistress', or a concubine, and Mrs was first used in 1615 before the name of married women, just like today, and to distinguish an unmarried woman from a child. So basically everyone woman who was not a mistress or a child was a Mrs.
The term Ms was not introduced until the late 1940's. It is then that Mrs was used to describe married women only. Not wanting to use Miss to describe concubines only, which are looked at much more poorly than they were in the 1600's, the term Miss was altered to refer to children,or young women. These young women could continue to use the prefix Miss or they could start using Ms.
The current etiquette is that women who are married use Mrs., and unmarried women, including children, use Ms. Miss can be used by anyone, married or unmarried. Whether or not you call a woman Miss, Mrs, or Ms, is up to them, if they haven't decided to be called Miss, then you should use the appropriate Mrs or Ms. If you have no idea what to call then I suppose the most polite thing to do is ask, or use the term Miss until they clarify it for you. Therefore it is correct to use Ms instead of Miss when you know an unmarried woman prefers to go my Ms rather than Miss.
Ms would be pronounced as mizz, Mrs as missis, and Miss as miss. How's that for confusion.
