As the name suggests, the Uninterruptible Power Supply is an appliance that is used to provide a continuous supply of power to an electrical device. It can supply the device even when the main source is temporarily shut down, as during a blackout. At such times, it provides the device with its own stored energy.
Thus, it ensures that the device continues functioning even if the primary source of electrical supply is not able to provide the power required for its functioning. This is very important to improve efficiencies of workplace environment that rely largely on electric supply, such as an office that uses computers.
There are three main types of UPS, based on their method of providing supply.
Standby UPS, also known as the Offline UPS - This is the most basic type. It works in the following manner - As long as the external electrical supply is on, it remains inactive, though it keeps on charging itself.
However, if there is any ebb in the amount of power being supplied to the device, the UPS will take over and supply current itself. This will continue until the primary supply is back to normal. When that happens, the uninterruptible supply will go back into its standby mode.
On-line UPS - The on-line UPS draws current from the main source, but supplies the current from its own battery to the device that is connected to it. In this manner, it delegates the main source to a secondary position, and itself becomes the main source for the device.
Such a system will provide a continuous source to the device through its own battery, which is in turn charged from the external source. In case the main supply fails, the battery keeps providing power to the device without as much as a blink. To understand this, recall the way your laptop works when it is connected to AC mains.
As long as it is connected to the mains, it will draw power from there, but if you shut down that switch, the laptop will still remain functional, because then its internal battery takes over completely.
Line Interactive UPS - This is a modified form of the original system, in which there is a regulator to keep the current coming into the device at a steady flow. You can consider this as a normal standby type with a filter attached.
Typically, it functions by drawing some part of current from the AC mains to charge its own battery, and uses the rest of it to power the device attached. In case there's a failure, the line connecting to the AC mains is switched off and the battery takes over in powering the device.
The general perception is that an UPS provides power to an electrical device in case there's a blackout, and that's the only function people attribute to it. However, the UPS performs several other functions. It not only powers the device when there's a failure, but it can also keep the voltage difference across the device, at a constant rate.
That means, if there is an unexpected fall in the current flowing into the device or, for that matter, even an unexpected surge, it will take care that the voltage difference across the device is still maintained constantly. For sensitive devices like computers and air conditioners, this is a very important benefit.
If you think by the principle of working, anything that provides an unhindered supply to an electrical device can be called an UPS.