There are 18 civilizations in Civ5 vanilla and 4 additional as downloadable content, each with its own leader and unique unit. In addition, every leader has a semi-unique "personality", consisting of traits that determine their in-game behavior.
For example, Elizabeth of the English, a historical naval power, emphasizes a pursuit of naval dominance and the choices she makes reflects this desire. However, in order to prevent leaders from becoming too-predictable, these traits have the potential to be changed slightly and weighted differently before a new game begins, keeping diplomacy fresh....
Regardless of Leadership or
nationality, all rival civilizations in the Civilization 5 are
dangerous. Civilization 5's take on world politics is decidedly
bloodier in nature than its recent prequels. Some civilizations act
more aggressively than others, but any of them will attack you if they
smell weakness. Beware.