An "Instruction Set" is a collection of commands that a computer's central processing unit (CPU) can understand and execute. These commands tell the CPU how to perform specific tasks, such as arithmetic operations, data movement, and control flow. Each type of CPU, like those from Intel or AMD, has its own unique instruction set, which defines the operations it can perform. Instruction sets can be categorized into two main types: CISC (Complex Instruction Set Computing) and RISC (Reduced Instruction Set Computing). CISC instruction sets have many complex instructions, while RISC instruction sets focus on a smaller number of simpler instructions. This design choice affects the performance and efficiency of the CPU in executing programs.