A Duke gains his title from a Duchy or Dukedom. A Duke is the highest ranking title in the British peerage system and comes from the latin word Dux meaning “Leader”. A Duke is styled Most Noble, referred to as His Grace and is called Our Right Trusty And Right Entirely Beloved Cousin by the King or Queen.
The wife of a Duke is a Duchess, his eldest son is a Marquess, his younger sons are Lords and his daughters are Ladies.
The son of a ruling King or Queen almost always gets made a Duke when they come of age as a matter of tradition. this is in addition to being granted the title Prince of Wales, the highest ranking royal title next to King or Queen. The current Prince of Wales, Prince Charles, is Duke of Cornwall.