It's sort-of "just DnD," only not. To understand, please endure this brief history lesson:
One of the best things that came out of 3rd edition DnD when it was released in 2000 was the Open Game License, or OGL. The OGL was basically a license that let other publishers use the info from the core DnD books and make their own rules, adventures, even complete alternate d20 systems like Mutants and Masterminds, Spycraft, and Iron Heroes.
One such publisher was Paizo, who specialized in a series of adventure paths under the name Pathfinder. They also published Dragon and Dungeon magazines. They were very successful, but their success depended on continued support for 3e via the OGL. When Wizards of the Coast announced 4th edition in 2007, it meant that 3e would no longer be supported. They also reclaimed their license for Dragon and Dungeon magazines. So with 4e, the two pillars of Paizo's success were about to come crashing down.
The OGL was still active, however (the wording of the OGL is such that it can never be rescinded). So Paizo came up with a fairly daring plan: They would essentially take the core rules from 3e and republish them, rebalancing the rules and adding a few new features. That way, there would still be a supported rule set for their Pathfinder adventure paths, which they could then continue selling. This became the Pathfinder RPG, which soon became the chief competitor with 4th edition DnD.
From the start, Pathfinder was very similar to the core 3e ruleset (hence the informal monicker "3.75e"), but has since differentiated itself with a number of supplements. By the terms of the OGL, all these new rules are Open Content, meaning among other things they can be viewed for free at Paizo's
Pathfinder Reference site, or this fan-made
Pathfinder SRD site.