Crusader Kings is a strategy game based on the Europa Universalis engine that focuses on the feudal kingdoms of medieval Europe in the time period of 1066 to 1453 A.D. The map is limited to Europe, stretching roughly to the Urals in the east and including parts of Persia and the Arabian Peninsula as well as the coast of North Africa.
One revolutionary feature in Crusader Kings is that the game is more about prestige and wealth than traditional empire building. To the player, the honour and power of the royal dynasty are more important than the expansion of the nation's borders (although that is of course still an important aspect of the game).
In Crusader Kings, players also have to deal with the feudal lords and bishops that rule the various provinces.
To retain the focus on medieval feudalism and chivalry, merchant republics like Venice, Genoa and the Hanseatic League are not playable nations; neither are the Mongol hordes nor the three Caliphates. The playable nations are limited to Christian monarchies, principalities and empires.
The goal of the game is to amass as much Prestige and Piety as possible for your dynasty.