The key differences between these three terms has to do with courtesy, age and often marital status. The honorific "Ms." is often applied to any professional woman over the age of 30, though there is no clear grammatical rule regarding age in this case.
Alternately, the title "Miss" is used for young women and girls, sometimes in place of their given name but more often as another honorific. In this case, it specifically applies to unmarried women of any age, but professionals normally prefer to use "Ms." Finally, "Mrs." is specifically applied to married women. Further, where "Miss" is a full word, both "Ms." and "Mrs." are abbreviations. Today, the use of the title "Ms." can also help to eliminate confusion or potential offense. It is considered good etiquette in almost every situation where a woman's marital status is unknown. In fact, many professional journalists, personal assistants and executive assistants rely on this unless or until the person in question makes a preference known. In any event, the term is not new, though some would have it believed otherwise. Each of these three had been used as a diminutive form of "mistress" until the abbreviated "Ms." fell out of favor, only to be later revived in the early 20th century.