An Off-line UPS bypass the AC power to output/load, when a power failure occurs it switches from AC power to its own power source (Battery)instantaneously (The switchover time is usually between 2 to 20ms). As the AC power is directly bypass to the load during that period the UPS is remain Offline, in the simple word the UPS is not working at that moment, When a power failure occurs it starts working so its called an offline UPS. At the time of bypassing power to the load, Offline UPS also charges the connected batteries during normal power supply. The Offline UPS use source power only during power problems and when the main power directly bypass to the output/load at that time the voltage of output very as per your input power. So its not good for your sensitive electronic devices.
An on-line UPS continuously powers the output/load from it's batteries while simultaneously replenishing/fill the batteries from the AC power.The UPS first converts AC power to DC to charge the batteries and the battery power is converted from DC to AC to power the output/load. As the battery power is supplied to the output/load at all times, it is known as Online UPS. Any change in the input voltage does not affect the output voltage so the output/load always gets clean power. Online UPS is recommended for sensitive electronic devices. online UPS protects against all common power problems so it is also known as a power conditioner and a line conditioner.