Different of online ups and offline ups?

In offline UPS the load will be directly connected to tha incoming AC power supply and the inverter will be always off until main AC supply is off or below or over the limits, while it's on in online UPS and load is connected to the inverter.

The main different between Online and Line interactive (or Off Line) is: the Switch from mains to Inverter, in the Online systems there are no switching or transferring, so the transfer time is zero.