WHAT IS THE DIFFERENCE BETWEEN ONLINE AND OFF-LINE UPS?

Offline UPS

Offline UPS supply grid power to electrical and electronic devices during normal power supply. It switches to battery power only during power failures and other power problems. As the grid power is bypassed to the load during normal power supply, it is known as Offline UPS. As the UPS is used only during power problems it is also known as Standby UPS. Offline UPS is also spelled as Off-Line UPS or Off Line UPS.
Apart from bypassing power to the load, Offline UPS also
charges the connected batteries during normal power supply. The bypassing of power results in exposing the load to electrical noises and surges. Therefore, they are not recommended for very sensitive and critical electronic devices. During power failures and other power problems, they switch from grid power to battery power fast enough to prevent computers from rebooting. The switchover time is usually between 2 to 20ms.

Online UPS

Online UPS supply power to electrical and electronic devices from batteries all the time. The UPS first converts grid power to charge the batteries and then discharges the batteries to power the load. As the battery power is supplied to the load at all times, it is known as Online UPS. The grid power is converted from (Alternating Current) AC to DC (Direct Current) to charge the battery and the battery power is converted from DC to AC to power the load. Therefore online UPS is also known as Double Conversion UPS.
As Online UPS always provides power from the battery, the load always gets clean power regardless of any power problems. Therefore, Online UPS is recommended for sensitive and critical electronic devices. Online UPS provide a layer of insulation or electric firewall. Any change in the input voltage or frequency does not affect the output voltage and frequency. Therefore, the load is shielded from all power problems.

source- sa dop