Input unit: These are used to read data and transfer to primary memory contained in CPU through keyboard or floppy disk or mouse etc.
Central Processing Unit (CPU): This consists of three divisions. They are:

1) Primary memory unit: This memory unit allows computer to store the data orinstruction read by input devices or data obtained by calculations in the program.

2) Control unit: This controls unit directs the operations of all units. This unit decodes and executes the instructions given in the program.

3) Arithmetic logic unit: This performs basic arithmetic operations like adding, subtracting, multiplicating, dividing and uses trigonometrical functions. It can also perform logical operations like comparing two variables, justifying the conditions. It can take many logical decisions.
Secondary memory or storage unit: These are external storage units and store data or program permanently on magnetic disk and magnetic tape.

Output unit: Output unit is used to take data from primary memory and writes it on output devices like video display unit or printer etc. All these physical units and various other equipments in which the computer is made of is called ‘hardware'. The sequence of instructions given to a computer to solve any problem is called ‘software'.