An online UPS is a type of Uninterruptible Power Supply (UPS) that always provides power to form DC to AC conversion even when the Grid input is available. In this case the Output load is totally protected from any variation or fluctuation in input power. This is used for various loads application where the transfer time gap of mili seconds cannot be accepted for running loads.
An offline UPS is a type of Uninterruptible Power Supply (UPS) that provides power to connected equipment by converting direct current (DC) to alternating current (AC), but when grid is available at input, the input power is transferred directly to load by ensuring the all protections of the load . These types of system are generally used for less sensitive load and home application.