Stands for "Reduced Instruction Set Computing,"and is pronounced "risk." It is arguably the fastest and most effiecient microprocessor technology available today. The RISC architechture is an improvement upon the CISC (Complex Instruction Set Computing) architecture used in the original Intel Pentium chips.
In 1974, John Cocke of IBM Research was working on making a faster version of the CISC chip and came up with a design that significantly reduced the number of instructions need for performing computations. The new design was not only faster than the CISC architecture, but the chips were also smaller and less expensive to manufacture. Motorola's PowerPC chips (such as the G4 in Power Macs) are the most widely used RISC-based chips. Intel has slowly been integrating RISC technology into its chips, but they still are mostly CISC-based.