NationStates is a game in which you create your own nation to rule. Then you decide on different political issues, and the nation will develop accordingly. For example, you decide what kind of political and econmical freedoms you like in your nation.
From the FAQ:
NationStates is a nation simulation game. You create your own country, fashioned after your own ideals, and care for its people. Either that or you deliberately torture them. It's really up to you.
There's also some interaction with other nations, but mainly this seems to be a single player game.