RISC and CISC
Beginner
Two styles of command list. RISC uses only small, simple commands. CISC also has big commands that do several jobs at once.
Novice
RISC (reduced instruction set computer) has simple instructions of fixed length, and only loads and stores touch memory. CISC (complex instruction set computer) has many instructions of varying length, and some combine a memory access with arithmetic. Arm and RISC-V are RISC; x86 is CISC.
Expert
The 1980s argument: simple, regular instructions make pipelining and compilers easier at the cost of more instructions. Today x86 cores translate instructions into RISC-like micro-ops, and measurements find little intrinsic performance or energy difference between the styles.
Explained in Instructions and pipelines (Architectures).
See also: Instruction set architecture (ISA), Iron law of processor performance.