Uninterruptible Power Supply - UPS

An UPS is a device that provides battery backup when  

the electrical power fails or drops to an unacceptable voltage level. Small UPS systems provide power for a few minutes; enough to power down the computer in an orderly manner, while larger systems have enough battery for several hours. In mission critical data centers, the UPS systems are used for just a few minutes until electrical generators take over.

The UPS also provides protection from power surges. A surge protector filters out surges and spikes, and a voltage regulator maintains uniform voltage during a brownout, but a UPS keeps a computer running when there is no electrical power. UPS systems typically provide surge suppression and may provide voltage regulation.