SMPS: Switched-mode power supply.
For running the computer the required electricity.
Popularly the electricity is available in the term of AC or DC.

SMPS is also used for transferring electricity as required by the particular of the computer.It transferred electricity on a balanced basis.
SMPS is like a square that transfers the electricity in many parts like the motherboard, DVD Writer, HDD.
The SMPS has four components
Input rectifier,
Inverter
Voltage converter
Output regulator
Why should we use SMPS?
As seen by engineers many places have electricity issues that are not available in constant mode. SMPS is used for regulating input-output for running the electric component of the computer, its saves the computer from high and low voltage.