What’s the difference between ‘Miss’ and ‘Ms’?

Miss is a title for an unmarried woman or girl, especially one under the age of about 30 (e.g., ‘Miss Jones’). It cannot be used for a married woman. It is sometimes seen as slightly old-fashioned, since it defines the woman by her marital status.

Ms is a title for a woman whose marital status is unknown, for an older unmarried woman, or for any woman in a context where you don’t want to emphasise the woman’s marital status. It’s intended to be neutral, in that it can be used for married and unmarried women alike much like ‘Mr’ can be used for married and unmarried men.