When WotC announced the fourth edition of D&D, they took back the publication rights to the magazines in order to create exclusively online versions. Pathfinder was created by Paizo Publishing, a group that was split off from Wizards of the Coast in 2002 to publish Dragon and Dungeon Magazines. After 10 years of publication, the first edition of Pathfinder was phased out in favor of the Second Edition - this edition continues to evolve the ruleset towards incorporating more simplicity while still finding a way to give more options for creating characters and adventures to challenge them.
In short, it's D&D for those who disliked the changes found in D&D's Fourth Edition (or who liked 3.5 and wished to remain compatible with its rules). It is a tabletop game based upon the rules of Dungeons and Dragons' 3.5 Edition, but expands on them, making additional rules, rebalancing classes, and simplifying some aspects. The Pathfinder RPG (in its first edition) has been called a Spiritual Successor to the 3.5 Edition of Dungeons & Dragons.