CPU (Central Processing Unit):
CPU or central processing unit is an electronic circuitry that carries out the instruction given by a computer program. CPU execute instruction by performing basic arithmetic, logical, control and I/O operations as required per instruction. CPU is considered to be the brain of the computer. The speed and efficiency of a computer mostly depends that of it's CPU.
Suggesting Reading:
Functions of CPU:

- Controlling all other parts of the machine and sending timing signals.
- Transferring data between memory and I/O devices
- Fetching data and instructions from memory
- Decoding instruction
- Performing arithmetical and logical operations
- Executing programs stored in memory
- Performing communication among the I/O devices etc.