If you was to me formal but the woman is unmarried or you don't know if she is married, you can't use "Mrs.", so you use "Ms." "Mrs." is for married women and "miss" is for young women. Here is an example: A 15 year old girl with the last name Smith would be "Miss Smith" A 40 year old married woman would be "Mrs. Smith" A 40 year old unmarried woman would be "Ms. Smith"

When you want to address a woman whom you do not know, you have to use Ms. eg=>Excuse me ms.?