Queen Elizabeth II has bestowed the official royal titles onto Prince William and Kate Middleton. She will, of course, take the appropriate female titles once they are married, making them the Duke and Duchess of Cambridge. Duke is the highest rank in the British peerage.
But that is not all! Prince William will be the His Royal Highness Duke of Cambridge, Earl of Strathearn and Baron of Carrickfergus. That makes Kate her Royal Highness Duchess of Cambridge, Countess of Strathearn and either Baroness or Lady of Carrickfergus. (Reports differ. Typically Barons have Baronesses, but it could be Baron and Lady.)
These titles link them to Britain (Cambridge), Scotland (Strathearn) and Northern Ireland (Carrickfergus). The new titles are a wedding day tradition for the Royal family. The Dukedom of Cambridge was created in 1801, but has not been used since the death of Prince George, who was the 2nd Duke of Cambridge, in 1904. Interestingly, Prince George also an army officer who famously refused an arranged marriage and instead married a commoner he was in love with, Sarah Louisa Fairbrother.
Earl of Strathearn has not been used as a title since the 1943 death of Alastair Windsor, the 2nd Duke of Connaught and Strathearn. And Baron of Carrickfergus has not been used since the 1883 death of George Chichester, 3rd Marquess of Donegall, who was the first and only Baron of Carrickfergus, named thusly by Queen Victoria in 1841.
Kate Middleton is one of many women to historically be called Countess of Strathearn and the third Baroness/Lady of Carrickfergus. She is not technically Princess Catherine, however. According to protocol, she would only be Princess Catherine if she was born a princess. Instead she is Princess William of Wales, as she takes her husband’s name instead.