Imagine this,
You are working on that all important presentation to be sent to the client and the power goes off.
You spent a fortune on your computer equipping it with the latest equipment, can you leave it to the vagaries of power fluctuations.
This is where a UPS comes in

UPS other wise known as Uninterrupted Power Supply works by taking the best of two devices, a stabilizer and an inverter.
Thus a UPS acts as

As mentioned above a UPS regulates the voltage and provides backup during a power failure. A stabilizer regulates voltage but does not provide backup; while an inverter provides backup, but does not regulate the voltage. Moreover a UPS is calibrated for PC usage while a stabilizer/inverter are not. This means that a UPS can be bought and directly used with a PC making it a Plug and Play device to save your computer against power failures & voltage surges.

The Surge Suppressor and the Filter ensure that when the power is drawn from the mains there are no undue fluctuations or spikes passed to the computer, which can harm it. In the event of power failure, computer is moved to battery-supplied power by the Transfer switch. The Inverter converts the DC voltage supplied by the battery into AC voltage and ensures that there is a uniform voltage, which is given to the computer.
Apart from the standby UPS shown above, another type of UPS called the line interactive UPS is also used. This type of UPS is very expensive and is mostly used in cases where saving data is very critical.

A UPS is a robust device seldom requiring any maintenance or upkeep. The only consumable that is used in a UPS is a battery, which over the course of time might become less effective. Batteries are available specifically for UPS and only these should be used. Using generic/automotive batteries should be avoided at all costs since they may spoil the electronic parts of the UPS.