The UPS (Uninterruptible Power Supply), is a device that offers an extra protection to your equipment. In case of an electrical mains power interruption, the UPS continues to feed your computer during the time needed to save your work.
This feeding is provided by a battery that is always being charged while the mains network is operating according. This battery has an autonomy which will hold the electrical feeding somewhere between 10 and 15 minutes. Therefore the UPS shouldn't be used to work while there is no mains, but to have the opportunity to save your work and turn the computer off.
That's why it is suggested that you shouldn't connect other peripherals to the UPS, such as printers and scanners. You should connect only the video display.
No breaks are classified in two types: off-line and on-line. The off-line UPS are cheaper and present a delay in their in their switching. When mains fails, the UPS takes a time, typically 16 ms, to detect the lack of energy and to connect the battery. Though a small delay it can affect the operation of more sensitive equipment. A quite common type of UPS is the so called line interactive. This type of UPS offers a shorter interruption, typically 6ms, and has an embedded voltage stabilizer.
The on-line UPS don't present any interruption in the mains, being therefore the better than the off-line UPS. There are however two types of on-line UPS: parallel on-line and series on-line.
In the parallel UPS their batteries and the mains voltage are connected simultaneously to the equipment outlet. As the battery is always connected to the UPS outlet, there will be no voltage interruption. However, as mains power is always connected to the UPS outlet, any fluctuation in the mains voltage will be present in the UPS output, and consequently in the computer. Therefore we say that in this type of equipment the output is not insulated from the input.
This insulation is attained in the series UPS, which is the best type of available UPS - it is what we call the true UPS. In this equipment the computer is fed only by the battery and only by the battery. When mains fails, there is no type of interruption. It is as if the electrical voltage present at the UPS input is used only to load the battery when it goes low. The UPS output is completely insulated from the input. With that, any problem in the mains power network (fluctuations, noise, etc) will never affect the computer connected to the output of this type of UPS.
If you're trying to buy a UPS to protect your computer, the question is to check the prices on the market and chose which equipment offers you the best cost/benefit relation to you.