A UPS is a Universal Power Supply. It is an extension cord that has a battery built in it. It is actually a box that plugs in your electrical outlet and then has several outlets for you to plug devices in for their electrical current.
The UPS supplies a steady rate of electricity to the devices plugged in it. If the power coming in your house or business drops or cuts off the UPS has a battery that continues to supply the electricity for a small amount of time, usually up to about 10 minutes. If the power amount varies the UPS keeps the power at a steady rate.
When I was in Brazil on a mission trip a number of years ago we had to have UPSís on the computers as the power supplied by the electric company was very inconsistent.
The UPS also works as a surge protector in that if an excess amount of power comes down the wire, it stops the excess from going to your computer or other device. Lightening or power line breaks can cause surges even a distance away from your location.
Using a UPS will help protect your computer equipment from being damaged by bad electricity. You can also buy extension cords that have surge protectors built in them to stop the surges but if you suffer a brown out (lowered amount of electricity supplied to you, occurs in some places in the summer sometimes) or power loss you are not protected.
I would encourage everyone to use at least a surge protector on their electrical equipment. If you do have a UPS do not plug laser printers in the battery protected outlets as they will draw the battery down quickly. Just put them in the surge protected outlets. The computer and monitor are what you want in the battery protected outlets.