So far, computers with Intel CPUs can't run Mac programs at all. The difference is one of computing philosophy. CISC chips like the Pentium sacrifice some raw speed in favor of having a complete set of built-in instructions on the chip. RISC chips such as the PowerPC contain far fewer instructions but can execute their tasks much faster. But all CPUs are so fast today that you don't have to worry about the architecture unless you're specializing in one of these applications.
Some computers use parallel processing to get the job done.
This refers to systems that use multiple microprocessors working in tandem. Multiprocessor computers are generally used as servers that manage computer networks and large Web sites.