Ruling Steward was the title used for the rulers of Gondor after the loss of the last King, Eärnur. The first of the Ruling Stewards was Mardil Voronwë. Before him the Stewards of Gondor were only the advisors to the King of Gondor. The title was hereditary, and was carried on through twenty-six generations to Denethor II, Ruling Steward at the time of the War of the Ring. His son Faramir gave up the rule of Gondor to Aragorn II, the rightful Heir of Isildur. Faramir then took up the traditional role of a Steward of Gondor as the advisor to the King.
See the Line of Ruling Stewards.