Fat Princess is a 32-player multiplayer game split into multiple modes, though you'll likely spend most of your time in Rescue the Princess. The idea behind the mode is similar to Capture the Flag, only instead of trying to trek across the battlefield with a piece of fabric on a stick, you're hauling game's title character back to your castle. If you can manage to keep her there long enough, your team wins.
However, matches aren't nearly as simple as they seem. The advantage to jumping into the game a little later is the knowledgeable player base. Though not huge (I never had a problem getting into a game), games were filled with players who loved the game and knew how to play. Rather than everyone running for a combat class, players know how to use special abilities to help their team out. For instance, the Worker class may not see like an exciting job, though his resource-capturing ability forms the backbone of any sound strategy.
So why the "Fat" in the title? In order to make sure your princess isn't kidnapped, you'll need to find pieces of cake scattered across the battlefield and feed them to her. The more cake she eats, the bigger she gets. Bigger princess equals more soldiers required to carry her. It's not the most PC of game mechanics, but helps illustrate why teamwork is so important.
There's also a single-player game, "Legend of the Fat Princess," which basically serves as a quick seven-game tutorial of sorts. It collects the various play modes and shows you how each are played. There's also "Gladiate," where you choose a job and fight waves of enemies. Both a nice additions and will quickly get you up to speed. However, Fat Princess is first and foremost a multiplayer experience.