If I'm not mistaken, that refers to Pathfinder Fantasy RPG (which is also the first thing that comes up if you search Google for 'PFRPG'). It started as a D&D 3.5 offshoot, adjusting and correcting many of 3.5's shortcomings. Perfect for anyone who didn't like the sweeping changes made in D&D 4th edition (it's been colloquially referred to as '3.75'). It's since grown into quite a contender in the industry.

-- Ben