Difference Between a Duchess and a Princess?

I've been reading so much about Prince William and Kate Middleton's  wedding coming up at the end of the month.  Once Kate, a commoner, says "I do", she will be an official princess.  I also read that Duchess Sarah Ferguson was NOT on the guest list, while her two daughters, also with the title princesses was.  What?  What makes a princess different from a Duchess ?

While both duchesses and princesses are royalty, and princesses technically outrank duchesses, the relationship between the two titles is not always clearly defined.

Princesses are usually the daughters or granddaughters of a king or queen. Traditionally, a "commoner," or a woman possessing no royal rank, can gain the title of princess by marrying a prince, with the possibility of later becoming a queen.

A duchess is the wife or widow of a duke, or a woman who equally holds the rank of duke in her own right, according to "The Index to Main Families, Persons, Places and Subjects in Egle's Notes and Queries" (John C. Francis, 1887).

In European nobility, the duke is the highest rank below the monarch, or king. The title of duke was first introduced by ancient Romans, who used it to describe tribal Germanic and Celtic war leaders, according to "A History of England Before the Norman Conquest" (London Studio Editions, 1993). During the Middle Ages, dukes ruled over provinces and were the highest-ranking peers of the king in feudal monarchies.

A princess or queen can also have the title of duchess and vice versa, according to "Lords of the land" (M. Joseph, 1984). For example, when Sarah Ferguson married Prince Andrew (Queen Elizabeth II's second son), she became a princess. And because Andrew also held the title of Duke of York, she became the Duchess of York.

And sometimes, women can even be dukes. Queen Elizabeth II, who was a princess for 26 years before becoming the current Queen of England, also holds the titles of Duke of Normandy in the Channel Islands and Duke of Lancaster in Lancashire.

A princess loses her royal title if the couple divorces. The same rules apply to the title of duchess.

There you have it...in a rather large complicated nutshell.  So, basically, interchangeable.  Most little girls dream of being a princess someday, swept off their feet by a handsome prince on the back of a beautiful white horse...but for you women who are still hoping for that to happen, Lady Diana was quoted as saying "it's not all that it's cracked up to be".

via What's the Difference Between a Duchess and a Princess? | LiveScience.