Radios waves carry signals either by FM or AM. AM (Amplitude Modulation) varies the strength of the radio's carrier wave according to the sound signal to be transmitted. Thus the shape of the sound wave is impressed on the radio's carrier wave signal. Interior electronics in radio players on our side will then recover the original sound signal and that is what we hear on the radio. FM (Frequency Modulation) is different. FM varies the frequency of the radio's wave according to the signal that needs to be transmitted. Radio players that receive the signal will unscramble the changes in frequency and convert them into electric signals that run the speakers so you can hear the sound. AM has a lower frequency then FM, and so the lower frequency AM can be partially reflected off layers of the atmosphere, in other words it can be transmitted for greater distances,especially at night. For example, people in China can sometimes hear Great Britain's BBC radio on AM.