Great People in Civilization 4
Every city can generate a Great Person depending on its speciality. There are five types of persons: artists, scienists, merchants, prophets and engineers. Usage of specialists gives some 'probability points' to the city in which some Great Person can be born. For example, if you have artists (aka Elvises) then expect Great Artist to appear.
Great People are just regular units with the exception that you cannot build them intentionally. They are visible only to the controlling player and can move at a speed of 2 squares per turn. Two or three Great Persons can start a golden age for the nation they belong to. (Note that in civ4 more then one Golden Age is possible.) In addition, every Great Person can accomplish some 'special mission' according to their 'nature': Scientists can rush discover certain technology, Prophet can build shrines and temples, while Artists can give you a culrure boost. All Great People in Civilization 4 have real historical names like Leonardo Da Vinci, Shakespear or Michelangelo.