Ms. is a general title that does not indicate marital status but is still feminine. Mrs. is a traditional title used for a married woman. Miss is a traditional title used for an unmarried woman. Mx. is a title that indicates neither marital status nor gender.
Generally speaking, it is considered proper etiquette to use Mrs. to refer to married women, Miss to refer to unmarried women and young girls, and Ms. to refer to a woman of unknown marital status or when marital status is irrelevant.