The Pathfinder Roleplaying Game is a fantasy role-playing game that was first published in 2009 by Paizo Publishing. It extends and modifies the Revised 3rd edition Dungeons & Dragons (D&D) game rules published by Wizards of the Coast under the Open Game License. Pathfinder RPG is intended to be backward-compatible with D&D version 3.5.
The Paizo Publishing company had been publishing Dungeon and Dragon magazines, which were about the Dungeons & Dragons (D&D) role-playing game. Paizo was publishing under contract with Wizards of the Coast, who held the rights to D&D. Wizards of the Coast chose not to renew the contract in early 2007, and Paizo began publishing the Pathfinder periodical line as a replacement. In August 2007, Wizards of the Coast announced the pending release of the 4th edition of Dungeons & Dragons, which replaced version 3.5. Many of the staff at Paizo were concerned about the more restrictive Game System License the 4th edition was being released under. Instead of continuing to support D&D, Paizo released Pathfinder as a modified version of the 3.5 edition game, under the Open Game License used by the older version. Announced in March 2008, the Pathfinder RPG was designed over the course of a year using an open playtest model, where players could try the system and post their feedback on Paizo’s website
Check out the Pathfinder SRD.