UPS is the acronym for uninterruptible power supply. It is a system that ensures sensitive electronic devices, such as servers and PCs, continue to operate if and when the main power source goes out. A UPS normally provides enough power for a few minutes to allow you to save your work before the computer shuts down.
A UPS system gives instant protection from power failures through the power stored in its batteries. While it may be possible for a standby generator to automatically start up when a power failure occurs, there is a short delay before the generator provides the power needed to keep your PC running. This is where a UPS system comes in since it provides you enough power while your standby generator is starting up.
It is also important to take note that a UPS system should not be considered as a replacement for a standby generator since it only provides enough power for a few minutes. A UPS system is normally used together with a standby generator in data centers since it fills in the gap in between the interruption of the main power supply and the replacement power provided by the standby generator.
Many UPS systems feature a software component facilitating the automatic backup and shut down of your PC whenever the main power source is interrupted. This feature is useful if you are not close to your computer when a power failure occurs.
The most common type of UPS system used in the market is an online UPS system. It is made up of a charger or rectifier, power inverter, and batteries. The power that passes through the charger and inverter is in the form of alternating current (AC) before it is stored in the batteries as direct current (DC). Whenever the main power source is interrupted, the DC power is converted into AC before it is used to power the devices connected to the UPS system.
References:
http://www.webopedia.com/TERM/U/UPS.html
http://www.businessdictionary.com/definition/uninterruptible-power-supply-UPS.html
http://www.amps.org.uk/what-is-ups-
http://www.wikiforu.com/2012/07/classification-types-of-uninterrupted-power-supply-ups.html