There are 34 civilizations and 52 leaders total in Civilization IV. Some civilizations have three leaders, others have two, and a few have just one. 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 usually 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). Every civilization has their own unique unit and unique building, both of which give an additional bonus.
Free Combat I promotion for melee and gunpowder units. Double production speed of Barracks and Drydock.
+2 culture per city. Double production speed of Library, Theater, and Colosseum.
+2 health per city. Double production speed of Granary and Harbor. 25% faster production of Workers.
+1 commerce on plots with at least 2 commerce.
Wonder production increased 50 percent. Double production speed of Forge.
Civic upkeep reduced 50 percent. Double production speed of Lighthouse, Factory, and Courthouse.
Great People birth rate increased 100 percent. Double production speed of University.
No anarchy. Double production speed of Temples and the Cristo Redentor wonder.
Great General emergence increased 100 percent. 50% faster production of Settlers.
Free Drill I and City Garrison I promotions for archery and gunpowder units. Double production speed of Walls and Castle.
+1 happiness per city. -25% XP needed for unit promotions. +1 happiness from Monument and Broadcast Tower.