Taken from the Wikipedia article:
Bluetooth is a short-range wireless technology standard used to exchange data between fixed and mobile devices over short distances and to build personal area networks (PANs). In the most widely used mode, transmission power is limited to 2.5 milliwatts, giving it a very short range of up to 10 meters (33 ft). It employs UHF (“Ultra high frequency”) radio waves in the ISM band, from 2.402 GHz to 2.48 GHz. It is mainly used as an alternative to wired connections to exchange files between nearby portable devices and connect cell phones and music players with wireless headphones, wireless speaker systems,car audio, and wireless transmission between TVs and soundbars.