Off-line (VFD) systems are usually low cost products designed for a simple one or two user PC installations, they offer little or no protection against most supply problems and really only give support for short-term power loss. The load is fed from the mains during normal operation, hence 'off-line'. On-line (VFI) UPS systems are regarded as 'high-end' and employ more sophisticated technology which uses a rectifier and inverter, hence 'double-conversion'. This effectively isolates the load from virtually all types of power supply problems.
What are the differences between 'Online' and 'Offline'?