MIDI stands for Musical Instrument Digital Interface. It's a standard protocol for saving and retrieving music information in digital format. Notes, their volumes, and the types of instruments to play them are stored and retrieved as numbers.

Originally, a MIDI Sequencer read MIDI data from a file, user input, or a MIDI keyboard, and passed that data to a MIDI Synthesizer, which contained the instrument sounds.

The MIDI Specification specifies 128 General MIDI Instruments and 16 MIDI Channels to put them in. Microsoft Windows has a built-in musical instrument synthesizer with all the General MIDI Instruments, but they don't sound especially realistic.

The SoundFont specification was created in 1996 by E-mu Systems to store instrument sounds, and it was used for years. There were some pretty good SoundFonts that sounded better than the built-in Windows synthesized sounds, but there wasn't any encryption for the digital WAVE data in a SoundFont.

To make instrument sound files more secure, Steinberg Media Technologies created the VST plug-in specification. This allowed creators of instrument sounds to keep control of their sounds in a VST plug-in, which could store them in any form they desired. The latest instrument sounds are now in VST plug-in format. Some are recorded (sampled) and some are synthesized.

With VST plug-ins, the 128 General MIDI Instruments aren't used because a VST plug-in can include up to 16 instruments in the 16 MIDI channels, and multiple VST plug-ins can be used together.

Music is still stored and played using the MIDI Specification.