For as long as anybody can remember, "Miss" was the official title for an unmarried woman, and "Mrs." was the formal title for a married woman. "Ms." is a little difficult because it may be used for either married or unmarried women. Nowadays, "Mrs." is used by most people when referring to an unmarried woman, while "Miss" is still used by some people when talking about an unmarried woman. It's important to note that even though these are the common words we use today, they're not always the case. You may hear other words being used instead. All in all, this is one of those words that doesn't have any real meaning on its own, but becomes clear once you look at the context.