The Central Processing Unit (CPU) of a computer is effectively the brain of the computer. It is a piece of hardware that executes instructions and manages the rest of the hardware. It has several component parts including the processor, main memory and cache memory. Program instructions and data move between the main memory and the processor using internal connections called system buses.
The CPU performs the following instructions in a continuous cycle:
Fetch instructions & data from main memory - Decode the instructions - Execute the instructions
It will also perform calculations and manage movement of instructions and data to and from peripheral devices.
This involves retrieving an instruction from a memory address. The address of the instruction is stored in a register called the program counter (PC)
The fetched instruction consists of an operation code (op-code) and maybe some data (or the address in memory of where to find the data) to act upon.
The decode step is the CPU working out what the op-code is telling it to do to the data e.g. store it somewhere / add it to something / subtract it from something etc...
To process from the memory or user input
All of the bits of the CPU that are needed for the instruction to be performed are connected and the instruction is carried out.For examples of the Von Neumann architecture see Little Man Computer or Reduced Instruction Set Computer