What makes someone from history 'Great', 'Heroic' or 'Feared'?

Why are people from history (e.g. Alexander The Great, Boudicca, Genghis Khan, Julius Casear) consided 'great', 'Heroic' or 'Pionate', ect. What attributes, technonlogies or knowledge made them remembered or famous/infamous? (This is for an ancient history report) >_>