The Flanaess is the continent the campaign takes place on. Most of the humans now on it are descended from refugees that fled a pair of magical cataclysms, the Rain of Colorless Fire and the Invoked Devastation. They were caused by a war between two ancient empires, now almost forgotten. The few ruins in the Sea of Dust are all that remain of them.
Almost nothing is known of the other continents of Oerth.