By default, new players are not members of any civilization, and start out as independent Nations. In CivWorld, there are 16 civilizations available to the players:

The civilizations are the major competitors in the game, each of which can comprise more than one Nation (player). Once a player has founded a civilization, any other player can join it. For game balance reasons, the maximum number of players in a given civilization cannot be higher than 20% of total players in the game. If a civilization is full, you need to find another one to join.

Independent players cannot be attacked, but they also cannot unlock most of the achievements nor have access to Civics and Wonders. A player can leave a civilization and become independent again, free to join another one.