Path Tracing is a pretty complicated process, and I'm having difficulty digesting it myself, but here's the gist: it's a method of rendering computer graphics using an algorithm that calculates all the light travelling to a specific point. If this system is combined with accurate textures it can produce imagery that is indistinguishable from reality. As Keanu Reeves might intone: 'Whoa'. Now, for the first time, there is an actual video game you can play that uses this technique.
Above we've posted a video that shows how it should look and play, but be warned a fairly beefy rig is required to stop things from chugging. There's also a 'grain' effect that is currently an unavoidable by product of the techniques being used to build the image.
Neogaf user -SD- has also uploaded some pretty impressive screens from the game. It looks pretty incredible.
You can download the game here.