CISC (Complex Instruction Set Computing) and RISC (Reduced Instruction Set Computing) are two contrasting approaches to computer architecture. CISC processors have a large instruction set with complex instructions that can perform multiple operations in a single instruction, while RISC processors have a smaller instruction set with simple instructions that perform only one operation per instruction. CISC processors tend to be more complex and have higher instruction execution times, while RISC processors are simpler and have faster instruction execution times.