Basically a big battery that will run your computer for a short time when the power goes out. There are two major types Online and Offline. An Online [UPS] is charging the battery constantly from the mains and drawing power from the battery to run the computer(s). This means that your computer is isolated from any power spikes or brownouts that might occur. An Offline [UPS] has a battery that is charged, and is switched inline when the power drops below a certain level. Usually protects you against brownouts, but has less protection from spikes. 

  

 a [UPS] is usually only there to smooth the power and handle short power outages (<10 minutes) and then allow the machine to shutdown cleanly. A [UPS] usually is not large enough to maintain power to the computer for an extended length of time (hours), unless your money:iq ratio is too high.