Fat princess is about saving the princess from the enemies and get you're own princess home. I've only played the demo, and on the demo you play online with people around the world.
The first thing you need to do, is to read about how everything works. (They don't allow you to play the game if you haven't read it.) Then you start playing the game! When the game is over, it's the same as in Cod Modern warfare 2, you can vote on which map you want to play, and you can switch team.
Fun game, for the big and the young.
I rate it: 9/10!