Frequency modulation synthesis (or FM synthesis) is a form of sound synthesis where the frequency of a waveform, called the carrier, is changed by modulating its frequency with a modulator. Most famously, the Yamaha DX7 synth features this type of synthesis. Learn how to sound design with this method and check out free alternative synth plugins.
Here´s a collection of ordered tutorials on how to do FM patches by Composing Gloves:
Here are 3 free FM VST synths (Click for more info and download links):
VF7 , Dexed , Oxe FM
Check out how close Dexed comes to the original DX7: