Online / Double-conversion UPS
An ONLINE UPS is the one in which the inverter always supplies power to the protected load and hence the same quality of power is ensured all the time. When mains supply is present, the inverter derives its power from the mians supplied rectifier and the backup batteries are also kept in charged state. When the mains fail, the source of DC power for the inverter section shifts to the battery without any break whatsoever in the output to the Inverter. It also bears all the vagaries and noise borne out of the mains supply and insulates the secure bus supply from it.
An ONLINE UPS is especially useful for sensitive and critical equipments/devices.