Most radios offer the listener the choice between the AM or FM bands. There are 4,650 radio stations that transmit on the AM band and 3,351 on the FM band. There are an estimated 500 million radio receivers in the US.
AM, standing for amplitude modulation, was the first radio band used. Later, FM or frequency modulation, was adopted.
In amplitude modulation, the sound causes the signal strength of the radio signal to vary. The radio receiver detects these instantaneous changes and converts it back into the sound. In frequency modulation, the sound instantaneously varies the signal's frequency. The variation in the frequency are picked up by a radio and converted back into sound.
The AM bands pack stations in a close spectrum with only a 10 kilohertz separation between signals. The FM band allows 200 kilohertz, or 20 times the spacing of AM, for the signals. With more space between the signals it's easier to receive the stations with high fidelity. Thus, FM bands have traditionally transmitted better quality sound than AM. However, engineers believe it's possible to transmit a relatively high quality AM stereo sound.