It should be said that, when purchasing equipment, 'MIDI Compatible' and 'General MIDI Compliant' are not the same thing. 'General MIDI Compliant' mean the device conforms to the MIDI interface standards as well as the MIDI instrument reference designs, whilst 'MIDI Compatible' just means (generally) that it has a MIDI plug.
The differnce is a 'General MIDI Compliant' piece of hardware will support all 128 standard (and maybe even the 512 extended) MIDI instruments; it also means that it can concurrently produce those instrument sounds -- i.e. I can have a guitar and piano at the same time. Under basic MIDI compatability, it just means it will send midi signals out the port and (sometimes) recieve them in a singular instrument pattern, so even though your computer is sending piano, guitar and drum signals to your keyboard, they will all be produced as if by one instrument.