1.The title 'Prince of Wales' may be possessed only by the eldest son of a Sovereign.
2.The title 'Princess of Wales' applies only to the wife of the Prince of Wales. The title cannot be used by the Sovereign's daughter, even if she is heir to the throne. For example, the Queen was known as Princess Elizabeth until she succeeded to the throne.
Why Camilla don't use the tittle. ?
3.The present Prince of Wales is 28st in the line, counting several who were never formally invested. He can trace his descent back through the Tudors to the original Princes of Wales, of whom the last native Prince of Wales was Llywelyn ap Gruffydd (1246-82).
4.Prince William is the elder son of The Prince of Wales and Diana, Princess of Wales.
5.The British Sovereign can be seen as having two roles: Head of State, and 'Head of the Nation'.