From the Pathfinder Society home page (http://paizo.com/pathfinderSociety):
Pathfinder Society Organized Play is a worldwide fantasy roleplaying campaign that puts YOU in the role of an agent of the Pathfinder Society, a legendary league of explorers, archaeologists, and adventurers dedicated to discovering and chronicling the greatest mysteries and wonders of an ancient world beset by magic and evil.
The campaign’s home base is sprawling Absalom, the so-called City at the Center of the World, that stands astride the great Inner Sea on the mountain-capped Isle of Kortos. A Pathfinder’s adventures explore the dark alleys and political intrigue of Absalom between far-flung travels to the most interesting and exotic locales in the world of the Pathfinder Roleplaying Game.
In Pathfinder Society Organized Play, you play a member of the Pathfinder Society, seeking fortune and glory all over the face of Golarion. At the same time, your character works for one of the competing factions, all with their own motivations and secret agendas.
Play is organized into Seasons, throughout which the actions and achievements of you and your fellow Pathfinders create an ongoing storyline. Each season consists of at least 28 Pathfinder Society scenarios (short, 4-hour adventures) set in a variety of exotic locations across Golarion.
For more information on Pathfinder Society, please download the free Guide to Organized Play.