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. In everyday usage, though, it typically comes down to personal preference.
Don’t let these similar-sounding titles fool you! Here’s how to use Ms. vs. Mrs. vs. Miss—and why the terms are important to women's history.