OPM is 4-op based on yamaha's synth keyboards. They have 8 channels. OPN is a version of OPM with 3,4 or 6 channels. OPN also have some extras such as PSG channels, DAC modes, or ADPCM playback. OPP adds some random noise mode. OPZ is used in the tx81z and has 8 waveforms.
Platforms
OPN: Genesis/Mega Drive
OPL are 9-channel, mainly 2-op, except for OPL3 which is 18-channel and optionally 4-op.
Platforms
OPL2: Soundblaster, Adlib
OPL3: SB16 and later, Adlib Gold