What is Low Power FM Radio?

Here is an explanation from the Federal Communications Commission:
The Low Power FM (LPFM) radio service was created by the Commission in January 2000. LPFM stations are authorized for noncommercial educational broadcasting only (no commercial operation) and operate with an effective radiated power (ERP) of 100 watts (0.1 kilowatts) or less, with maximum facilities of 100 watts ERP at 30 meters (100 feet) antenna height above average terrain (HAAT). The approximate service range of a 100 watt LPFM station is 5.6 kilometers (3.5 miles radius). LPFM stations are not protected from interference that may be received from other classes of FM stations. A construction permit is required before an LPFM station can be constructed or operated. The mere filing of an LPFM application does not authorize any construction or operation. LPFM stations are available to noncommercial educational entities and public safety and transportation organizations, but are not available to individuals or for commercial operations. Current broadcast licensees with interests in other media (broadcast or newspapers) are not eligible to obtain LPFM stations.