We always laugh in class, but could you imagine calling a woman "Mister García"(Señor García)?
These are the differences:
We use MISS (pronounced "mis") when we refer to un unmarried woman. (In French, we would say "mademoiselle")
We use MRS (pronounced "misis") when we refer to a married woman.
We use MS (pronounced "mis") when we refer to woman, regardless of her marital status. So, if we don't know if the woman is married or not, we should use MS.
We use MR when we refer to a man. This word comes from the ancient word "Master".
To sum up: