There are 18 civilizations and 26 leaders total in Civilization IV. Eight of the civilizations have two leaders while the other ten civilizations have one leader. Unlike Civilization III, in Civ4 the traits are associated with the leaders, not with the civilization. Each leader has two traits. Each trait provides a unique bonus and cuts the hammer cost of one or two city improvements by half.
In addition, each civilization has a unique flag and its units will speak in the civilization's language when clicked (like in RTS games).
NOTE: Information in this section is correct as of Civ4 v1.61.
