CS stands for Creative Suite and this is a type of software that is being offered by Adobe right now. The numbers that are used can make it easier for the different software packages and their versions to be distinguished. The CS4 was released a year and a half later than CS3 which means that it is the newer and more advanced version. One of the main differences in the interface.
There is an interface change with the CS4 as it would look more like the usual internet browsers as compared to how it looked like in CS3. The Nvidia CUDA technology can be used with the CS4 but not with the CS3.