sockpirate :
Well what exactly is it, if someone wouldn't mind an explanation for a simple mind! ha ha
PhysX is an API used to do physics calculations. There are some games engines built to be able to use PhysX in the games. Some of these options can add some special effects to a game. Nvidia cards, starting with the 8600 and newer have the ability to perform these calculations on the graphics card, which can do it much faster than on the CPU.
The part that confuses most people, the ability to have physX performed on the graphics card is not often used, even on games with a PhysX graphics engines. However, games which do allow for GPU accelerated PhysX, will almost always perform a lot better with an Nvidia card when the advanced PhysX effects are enabled. In most games the 20ish games which offer GPU accelerated PhysX, the effects aren't done that well, but a few are pretty nice looking. Batman AA is one such game which adds a lot of special effects that look cool.
Mafia 2, Mass effect, Sacred 2, Metro 2033 are all such games with GPU accelerated PhysX. There are a few others. I personally own 2; Sacred 2 and Metro 2033. Sacred 2 is kind of hit an miss with it's physX effects. Some look ok, some don't. Metro 2033's physX is hard to tell it's even on and slows the system down more than it's worth.