From 1975, when her father inherited the Earldom of Spencer and until her marriage in 1981, Diana was known as the Lady Diana Spencer. 



The title Earl Spencer was created in 1765 in the Peerage of Great Britain for John Spencer, a great-grandson of the First Duke of Marlborough. Lady Diana Spencer, later Princess of Wales, was the daughter of the eighth Earl.



The subsidiary titles of the Earl Spencer are: Viscount Spencer (created 1761), Viscount Althorp (1765), Viscount Althorp of Brington (1905) and Baron Spencer (1761). All titles save one are in the Peerage of Great Britain; the title Viscount Althorp of Brington is in the Peerage of the UK. The courtesy title of the eldest son and heir of the Earl Spencer is Viscount Althorp.



After her marriage to the Prince of Wales in 1981, until 1996, Diana was known as Her Royal Highness, The Princess of Wales. 


From 1996 onwards, she was known as Diana, Princess of Wales. 

The usage Princess Diana, though commonly used in speech and the media, is incorrect. Diana, Princess of Wales herself made a point of correcting people who used it.