MIDI = pure electronic data (nothing inside, no sound inside, no character, only pure data for later claculations, like digital notes).
FM = Frequency Modulation [FM synthesis] (e.g. a carrier-waveform signal modulated by another sine wave, which gives the music a special harmonic characteristic).
PSG = Prorammable Sound Generator (simple sines, rectangles, sawtooth, mixed over each other, back coupled etc. stuff of sounds).