There are 18 Civilizations in the game with which you can play, 18 of them at the same time (if you've got the system to handle that many!). For each of these Civiliations there are one or two leaders available, for a total of 26 leaders. For instance, if you decide to play with the French, you have the choice of playing as Napoleon or as Louis XIV. This affects gameplay as every leader has their own personality and characteristics (a la Civilization abilities in Civ3). When playing as Napoleon, you get stronger Units because he is Aggressive and cheaper wonders because he's Industrious, when playing as Louis XIV, you get free Culture because he is Creative. Every leader has two such traits. Aside from leaders and their personalities, each Civilization has a number of other properties that make it unique: a list of historic City names, a banner or flag which identifies a Civ's Units on the map, a Unique Unit which only that Civ can build.
The following leader traits exist in Civilization IV:
|Trait||Bonus||Double production speed of ...|
|Aggressive||Free Combat I promotion of melee and gunpowder units||Barracks, Dry dock|
|Creative||+2 culture per city||Theater, Coliseum|
|Expansive||+2 health per city||Granary, Harbor|
|Organized||-50% Civics Upkeep Cost||Courthouse, Lighthouse|
|Philosophical||Double Great People birth rate||University|
|Financial||+1 gold on plots with 2 gold|
|Industrious||+50% Wonder production speed||Forge|
Civilization IV: Warlords will feature two new leader traits, but which ones hasn't been announced yet. It also adds 6 new Civilizations.
See the Civilopedia for details on the Civilizations in the game.