While I thought it was, that brings up other questions:
Why are some prince's wives (such as Diana) titled Princess and other princes' wives (such as Fergie) titled Duchess?
Does the territory of which one is a prince determine that?
Will the older of Charles' sons (sorry, I forget which one is older) become The Prince of Wales (and his wife The Princess of Wales) on the death of Prince Charles?
The reigning monarch is entitled to determine who gets what titles. The spouse of a member of the royal family does not necessarily have a title unless the monarch gives them one.
A duke/duchess is the highest rank after a prince/princess (after a monarch).
On marriage the wife takes the title of the husband - hence Diana, Princess of Wales, Sarah, Duchess of York, and Kate, Duchess of Cambridge. Wales, York, Cambridge being the titles of their spouses.
Where it gets complicated is second generation of past monarchs Prince Edward, Duke of Kent and Prince Michael of Kent are both grandchildren of King George V, but only the eldest son inherits the titles of the father, so Edward DoK's wife is HRH Duchess of Kent, while as Michael has no actual title, his wife is merely HRH Princess Michael of Kent.
The title of Prince of Wales follows the oldest son of the monarch and Heir Apparent, so Charles retains it untl he becomes King, when William receives it. If however, Charles predeceases Elizabeth II, William becomes Heir Apparent but Prince of Wales.
(It's actually a little more complicated than that.)