Thank you very much Meredith! As someone with a few CPUs and GPUs, most of them idle right now, I can say it's a total waste of time and electricity, most of the time. :P The short answer to why GPUs are faster is that they are designed for parallel operations, and CPUs are designed for serial ones. This old article on the Bitcoin wiki summarizes it. Ignore the stuff about AMD vs. Nvidia, it's not necessarily the case anymore.