Input Unit
The input device or input unit of the computer system is used for feeding data and instructions to the computer. An input device allows users to communicate and feed instructions and data to computers for processing, display, storage and/or transmission.
·        Keyboards: Allow users to input alphanumeric data and commands
·        Pointing devices and game controllers: Allow users to direct application software and interact with graphical user interfaces
·        Audio and video devices: Allow users to capture sound and images

Output Unit
An output device or output unit is used to send data from a computer to another device or user. A display device is an output device that visually conveys text, graphics, and video information. Examples of an output device include monitors, projectors, speakers, headphones and printers.

Input/output Unit
Devices for communication between computers, such as modems and network cards, typically perform both input and output operations such devices are termed as Input/output Units or devices. The touchscreen is also a popular example of such devices.

Central Processing Unit
The Central Processing Unit (CPU) is the computer component that's responsible for interpreting and executing most of the commands from the computer's other hardware and software. A CPU is further categorised into following parts for a simple and basic understanding of its functioning: 
1) Arithmetic-Logic unit
2) Control Unit
3) Memory Unit

Arithmetic-Logic Unit (ALU):  The ALU is an electronic circuit used to carry out the arithmetic operations like addition, subtraction, multiplication and division. This unit carries out logical operations like greater than, less than, equal to etc. It performs the operation on the data provided by the input devices.

Control Unit: The control unit coordinates the activities of all the other units in the system. Its main functions are to control the transfer of data and information between various units and to initiate appropriate actions by the arithmetic-logic unit. Conceptually, the control unit fetches instructions from the memory, decodes them, and directs them to various units to perform the specified tasks.
Memory Unit:  The main memory is also called primary memory, is used to store data temporarily. Although the CPU is the brain behind all the operations in the computer, it needs to be supplied with the data to be processed and the instructions to tell it what to do. Once the CPU has carried out an instruction, it needs the result to be stored. This storage space is provided by the computer’s memory.