its a PCI card you put in your pc which is "supposedly" meant to improve physics in a game.
Its certainly not worth buying at present not a lot of game support, and it has yet to be proven to be a worth while purchase. So no do not buy it YET.
there's also a demo called cell factor which was meant to be physics card only, yet you can run it without one and from what iv seen there's very little improvement. Except it can handle fluids and cloth tearing.
nvidia have an idea to use a SLI config to allow one graphics card to work as a physics processor and ATI are looking at having their own version.
Its a new technology and whether or not it will have benefits are yet to be determined, many think unreal 2007 will be the time to see if it sinks or floats.
in conclusion Id wait
its expensive and not worth wasting money on yet. Wait for more games to support it and benchmarks. Also keep an eye out for nvidia and ATI trying something similar.