Basically, the computer part of your computer. Sometimes means just the processor, sometimes refers the the entire box.
The CPU contains the chip or main processor of a personal or desktop computer. It performs the basic tasks and functions of the computer, and is often called the "brains" of the computer. The CPU may also refer to the housing or motherboard that contains the chip.
The brains of your computer. In fact, the main silicon chip that runs the operating system and programs, and controls essential operations.