The term "MIDI sound" has often been used as a synonym for "bad sounding computer music", but this reflects a misunderstanding: MIDI does not define the sound, only the control protocol. This is probably a result of the poor quality sound synthesis provided by many early sound cards, which relied on FM synthesis instead of wavetables to produce audio. -wiki
MIDI (Musical Instrument Digital Interface) is a format which allows musical data files to be shared among various electronic instruments (hardware synthesizers and sample-based machines) and computers (operating systems, soundcards or embedded into code) by using a standard list of commands and parameters known as GM or General MIDI.
Technically MIDI is not music, that is, midi music cannot be recorded or heard as sound. MIDI is a small amount of data that contains event messages. Event messages can include timecode (bpm), notes, volume, aftertouch, pitchbends, vibrato, panning, program changes, event changes and system based messages.
MIDI is an open system for music production and recording because it allows any instrument, or sound, to be assigned to any MIDI channel. Allowing any number of sonic experiences, however this does not mean everyone will hear the same sounds because of the sounds rendered may differ from one operating system to the next. Two Korg synthesizers may sound similar, but a Korg (or any synthesizer) and soundcard may be very different.