Here is a little applet to illustrate how a signal is sent via AM or FM radio. The top line is a plot of the signal being transmitted. The sine wave below that is the carrier wave, whose frequency determines the radio frequency. The signal is used to alter, or modulate, the carrier wave, either adjusting its amplitude (amplitude modulation or AM) or adjusting its frequency (frequency modulation or FM). Drag the circled points of the signal plot between the red lines to change the amplitude there and watch what happens to the AM and FM waves.