A UPS is meant for supplying electrical power (typically mains voltage) when the grid fails - either completely or brown-outs. Some UPS types can condition the power so the voltage is in range of the nominal voltage (eg. 230V ±10%)

There exist three main types of UPSes (and multiple variations thereof). 
  * Online UPS: Grid power charges a battery the battery drives an inverter which drives the protected devices.
  * Offline UPS: Grid power charges a battery and the protected devices in normal operation. When the grid power fails the UPS disconnects from the grid power and the battery starts driving an inverter which delivers power to the protected devices.
  * Line-interactive UPS: The grid power goes to an inverter which either charges or discharges the battery, delivering a conditioned voltage to the protected devices.

The batteries in UPSes are typically some sort of sealed-lead-acid, and can be recycled - sometimes with a discount from the manufacturer. 

Further reading:

 * https://community.fs.com/blog/line-interactive-vs-online-vs-offline-ups.html
 * https://download.schneider-electric.com/files?p_enDocType=White+Paper&p_File_Name=JSII-5YQSBR_R1_EN.pdf&p_Doc_Ref=SPD_JSII-5YQSBR_EN