When you compare inverter and UPS (Uninterrupted Power Supply), followings are the differences between them.
Size and Purpose:
UPS normally has low power storage capacity compared to inverter which is used to run almost most of the electrical appliances while UPS is just needed to run one or two device like PC, Laptop or printer.
UPS directly converts the AC to DC and store the power in DC form in battery. When you are using UPS, power to your device comes from battery in form of AC (after conversion into AC from DC storage). When there is a power cut, your equipment does not feel it as battery keep on giving power to equipment without any interruption. This is why it is named UPS (Uninterrupted Power Supply). On the other hand, Inverter also working on the same principle but the battery capacity is far more than UPS and also there are sensors which keep on looking for whether battery gets AC power from mains or not. If suddenly, power goes off, sensors takes time to give feed back to battery and hence you will feel an interruption in case of inverter. This is why during power cut off, you will have to wait for few seconds/moments to get power from inverter.