Cuda = gpgpu: General purpose computing on graphics processing unit(s). All this means is that stuff you normally do on a cpu like video rendering will be transfered to the faster gpu for processing. (although you need special programs to use this feature)http://en.wikipedia.org/wiki/GPGPUhttp://www.nvidia.com/object/cuda_what_is.html
PhysX= This is nvidia way of speeding up physics in games and science programs. This make the programs act much closer to reality than before.
Example for games: instead of shooting at a window and doesn't break, well in with this (if the game programed for it) the window will break.http://en.wikipedia.org/wiki/Physx#PPUhttp://www.nvidia.com/object/physx_faq.html