There are basically three types of UPSs: Standby, Line-interactive, and On-line
Standby : (sometimes called "off-line") With this type of power supply, power comes directly from the A/C outlet until the voltage sags or the power fails. After the power sags or fails, a battery powered inverter almost immediately turns on to continue to supply you with power. Batteries are charged, as necessary, when direct A/C power is available. Even while power is coming directly from the A/C outlet, the UPS provides protection from voltage spikes and surges. Standby units are the low cost solution in the UPS family.
Line-interactive : Just like the standby units, line-interactive UPSs provide protection from spikes and surges, as well as supplying auxiliary power if a voltage sag or a blackout occurs. Unlike the standby units, line-interactive UPSs provide automatic voltage boost when the power dips, without accessing the batteries. This feature provides continuous lined conditioning, promotes longer battery life, and eliminates electronic "noise" that can cause minor application errors and loss of data.
On-line : On-line UPSs provide the highest quality of power protection by using a double-conversion technique. The UPS takes the incoming A/C power and recreates it by converting the voltage to D/C, then conditioning the power to eliminate noise, sags, or surges, and, finally, converting it back to A/C before it exists the UPS. Since the power runs continuously through the inverter, there is no transfer or switching time to battery mode in the event of a blackout.