UPS
1) UPS means Uninterruptable Power Supply.
2) It is an electric circuit (device) which instantly backs up power supply for a gadget. The gadgets works continues to work on smoothly and there is no damage to it.
3) It first converts AC to DC Power to charge the battery than Convert DC Power to AC Power (Inverter) and this AC power is supplied to Load. However, UPS monitors the input voltage level and processes it in terms of voltage regulations.
UPS= Battery charger + Inverter

Inverter
1) Inverter is a device which converts DC electricity to AC.
2) Inverter consist circuitry which converts AC to DC and stores in the battery. When power supply goes off, that DC power is converted back to AC and is transmitted to the respective electronic gadget.
3) Inverter converts DC power (stored in its battery) to AC Power supplied to the devices. Normally AC Power charges the battery .It uses relays and sensors to detect when to use DC power or AC Power, for DC power.