There really is a difference between an uninterruptable power supply (UPS) and a standby power supply (SPS), but common usage now designates a unit properly termed an SPS as a UPS. We call a unit of either sort a backup power supply (BPS), which neatly sidesteps the terminology problem.
A BPS comprises a battery and some supporting circuitry, and is designed to supply power to your PC for a short period if the utility power fails. This temporary reprieve allows you to save your work and shut down the PC in an orderly fashion. BPSes differ in the quality of the power they supply, how much power they can supply, and for how long they can supply it. BPSes also condition the utility power to protect equipment against spikes, surges, drops, brownouts, and electrical noise.