Duke of Orléans is one of the most important titles in the French peerage, dating back at least to the 14th century. It was attributed always to princes of the Royal Family and was thus a junior branch of the ruling house. Very often in French History, the Duke of Orléans had an important political role. The Orléans came to the throne with Louis XII (15th century) and Louis-Philippe (19th century). The descendents of the family are the current pretenders to the French throne, and the title has been used by several members of the House.
Note that in some sense, all heads of the House of Orléans since 1830 have been entitled to use of the title Duke of Orléans, whether or not they have done so.