Frequency modulation allows the output of one oscillator to drive the frequency of another oscillator. It can generate very complex sounds from very simple waveforms and is the basis of the Yamaha DX range of synthisizers and OPL range of synthesizer chips (used in Adlib and most other soundcards)....
Korg MS-10
This was the most basic, and consequently the most inexpensive, of Korg's MS series.
It featured a 32-note (F-C) keyboard and semi-modular design, in that you could reroute
the signal of some modules via patch cords, but you didn't have to use patch cords to
make a basic synthesizer sound. A monophonic synth, the MS-10 had one VCO, one
VCF, an LFO with multiple waveforms, an ADSR with hold controls, and knobs to
control pitch, portamento time, external signal level, resonance, and pulse width. It also
had one wheel, for pitch bend.