A UPS can be categorised into 3 main types:
OFFLINE OR STAND BY UPS
This is the most basic type of UPS and is used in areas where the utility power is fairly stable with the occasional outage (power failure). This UPS comprises of a battery charger, a battery and an inverter that has a square wave output. Utility mains feeds the load (computer etc.) directly until a power failure occurs. When a power failure occurs, the UPS inverter then switches on and produces a square wave output to feed the load.
LINE INERACTIVE UPS
This is a more sophisticated offline UPS. Some line interactive UPS have a sine wave output. In addition to the battery charger, battery and the inverter, this type of UPS has a built-in regulator to stabilise power fluctuations. It does not however, provide good regulation that certain electronic equipment requires, nor does it provide good input to output voltage isolation.
Unlike the 2 types of UPS above, the inverter of this UPS is always running and feeding the load. The output of the inverter is a sine wave. The utility mains feeds the battery which keeps the batteries charged and the inverter running. In the event of an outage, the inverter keeps running, being powered by the batteries. The regulation of this type of UPS is excellent and the output is isolated from the UPS input. This is the ideal type of UPS to provide power to critical electronic equipment.