The difference's between an inverter and a UPS are as follows
1) CHANGE OVER TIME: An off-line ups (the standard) switches to the batteries in 3 to 8 milliseconds, after the main power has been lost.
The typical Inverter changes over in about 500 milliseconds
2) SIZE: Off-line UPS's go up to about 2kVA
Inverters go to 16kVA, 3 phase(what i could find browsing)
3)VOLTAGE REGULATION: UPS's have automatic voltage regulation (AVR) set on average at 220v
Inverters output voltage is the same as the mains input voltage (South Africa = 230V)
An inverter can transform a voltage from say 12volts dc and up to 230v AC.
UPS means Uninterrupted Power Supply. If you use an UPS between your computer, and your socket in the wall, and experience a power failure, the UPS will act as a battery and give you some extra minutes work time to save your work, and turn of the machine properly. Without the UPS your machine would die immediately.
*note: the UPS is NOT a normal battery, so you cant put a battery between your computer and the mains.
2) The basic