UPS units are designed to provide power to your equipment in the event of loss of mains supply.  They sit between the mains and your equipment, and contain batteries that are kept charged from the mains supply.  In the event of loss of mains power, the unit supplies power from the batteries.

Obviously there is a limit to the amount of power stored in the batteries, and so the two things which determine how long the power will last for are the current draw of your equipment and the capacity of the batteries.

When choosing a UPS for Computer systems, several additional issues arise.  Because the cost of UPS units increases with their capacity, a unit is often chosen to provide the minimum run time that will allow an orderly shutdown of the equipment.  This is because the computer's data is usually considered to be more important than continued access.  Dropping the power to the computer when it is in the middle of processing can have catastrophic data and hardware results.  To help with this, most APC units have the ability to send a message to the computer when battery power is running out to tell it to shut down in a controlled manner.  This communication takes place using a cable or via a network, and works with software (provided with the unit) that runs on the server.