UPS stands for Uninterrupted Power Supply. When the main electric power supply fails, the UPS helps you to retain the electric power supply for sometime so that you can wind up your work without abruptly stopping it. Depending on the model that you use the power supply could be maintained for 20 minutes up. The UPS has a battery that is charged continuously when there is power supply. Once the power supply is cut the battery that is charged earlier supplies the power to the devices. This is the basic principle by which the UPS works.
The input to the UPS is AC, internally it is converted to DC through rectifiers and stored in battery, the power from the battery is converted back to AC through an inverter and this power is the output of the UPS. When there is a power supply you will be alerted by the UPS usually by some sound signals indicating that the power is supplied from the battery. If you are using an UPS for working with your computer, upon getting this sound signal you can finish up your work and shutdown your system before the power from the UPS goes down. Thus by properly shutting down the system you prevent any damage to the hard disk in your system.
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