A power source (battery) that has surge protection. The servers are plugged into the UPS to ensure that if there is a power failure the systems keeps operating for a limited period of time in order to allow to save data in RAM and to perform a controlled shut down. Many UPSs now offer a software component that enables you to automate backup and shut down procedures in case there's a power failure while you're away from the computer. A laptop is an example of something that has a UPS function (the battery)
Equipment (usually a bank of batteries) that offers short-term protection against power surges and outages. Note that UPS usually only allows enough time for vital systems to be correctly powered down.
A backup power unit that provides continuous power even when the normal power supply is interrupted.