Quoted from the Pathfinder SRD:
The Pathfinder Roleplaying Game is released under the Open Game License, meaning the core rules that drive the Pathfinder RPG system are available to anyone to use for free under the terms of the OGL. This compendium of rules, charts, and tables contains all of the open rules in the system, and is provided for the use of the community of gamers and publishers working with the system.
This compendium is NOT the official Pathfinder Roleplaying Game! Players interested in a user-friendly introduction to the Pathfinder system will want to purchase the complete Pathfinder Roleplaying Game Core Rulebook and the Pathfinder RPG Bestiary, which comes complete with more than 350 monsters to menace your player characters.
Both books contain hundreds of beautiful full-color illustrations and lots of descriptive text to explain and introduce key rules concepts necessary to fully enjoy the Pathfinder Roleplaying Game experience. Both the Core Rulebook and Bestiary are also available in affordable PDF editions."
Publisher, Paizo Publishing