FM Synths

Frequency Modulation 

Frequency Modulation is where an original audio sample/patch gets a modulating wave added to it to create new harmonics, frequencies and subsequently, new sounds. You never actually hear the modulator, but you do hear the effect on the original signal. Once the frequency of the modulator gets up into the audio range, you stop hearing vibrato and start hearing new frequencies appear in your carrier. The modulating wave would be on an oscillator so it would come in and out of audio range.The new frequencies that are created are called sidebands. They can appear both above and below the original frequency. The number, frequency and amplitude of the sidebands are controlled by modifying the ratio, as well as the amount of modulation. Even ratios (2:1, 1:4, etc) tend to make harmonic sidebands, where non-even ratios (1.53242643:1) tend to create out-of-tune, lo-fi, enharmonic sounds.

Here's an example of an FM Synth:

FM Synths: Korg M1, Yamaha DX7

FM is also applied in the use of radios. The radio station sends out a carrier frequency, whilst your car/device sends out a modulating wave (which sounds like white noise) so that when the two meet, they play the carrier frequency through your car/device.