the site you mentioned makes too many assumptions from its readers.
1) an offline UPS ("regular" UPS) just sits there until there is a power interruption. then the UPS will supply power to the computer. less expensive but test for power continuity before buying. some UPS models do not react fast enough to save your computer from the power interruption
2) the online UPS ("sinewave" UPS) will convert your power supply from AC to DC to (simulated) AC supply continuously. no problem with power suppy continuity, more expensive