There are 3 main types: Standby UPS, On-Line UPS, and Line Interactive UPS. More info @ http://www.buzzle.com/articles/uninterruptible-power-supply.html
A power supply is generally only converting the mains voltage down to a lower voltage suitable for low voltage equipment. It turns 120 v into 18v or 12v or 5v or any other voltage it is specified...
stanby device(baterry-powered circuit is used when AC input fails), an inline device(battery-powered circuit is used continually). A line-interactive device combines features of the two above.
It depends on the type of power supply. A power supply used in a desktop computer is generally supplied with AC (Alternating Current) and produces several levels of regulated DC (Direct Current)...