Taken from wikipedia
Musical Instrument Digital Interface, or MIDI, is an industry-standard electronic communications protocol that defines each musical note or event in an electronic musical instrument or show device such as a synthesizer, precisely and concisely, allowing electronic musical instruments, computers and other show equipment to exchange data in real time. MIDI does not transmit audio — it simply transmits real time digital data providing information such as the type and intensity of the musical notes and technical cues played during a performance.
All official MIDI standards are jointly developed and published by the MIDI Manufacturers Association (MMA) in Los Angeles, California, USA (http://www.midi.org), and for Japan, the MIDI Committee of the Association of Musical Electronic Industry (AMEI) in Tokyo (http://www.amei.or.jp). The primary reference for MIDI is The Complete MIDI 1.0 Detailed Specification, document version 96.1, available only directly from MMA in English, or from AMEI in Japanese. There exists a seperate implementation for mobile devices known as SP-MIDI (Scalable Polyphony MIDI) this can be adapted to particular hardware architecture on specific types of 3GPP mobile devices.
Almost all music recordings today utilize MIDI as a key enabling technology for recording music. In addition, MIDI is also used to control hardware including recording devices as well as live performance equipment such as stage lights and effects pedals.
The MIDI standard was first proposed by Dave Smith in 1981 in a paper to the Audio Engineering Society and the MIDI Specification 1.0 was published in August 1983.
MIDI allows computers, synthesizers, MIDI controllers, sound cards, samplers and drum machines to control one another, and to exchange system data. Though modern computer sound cards are MIDI-compatible and capable of creating realistic instrument sounds, the fact that sound cards' MIDI synthesizers have historically produced sounds of dubious quality has tarnished the image of a computer as a MIDI instrument. In fact, the MIDI specification itself has nothing to do with the quality of sound produced - this varies depending on the quality of synthesizer, sound card, and/or samples used.
A number of music file formats have been based on the MIDI bytestream. These formats are very compact; often a file of only 10 kilobytes can produce a full minute of music. This is advantageous for applications such as musical ringtones in mobile phones, and some video games. The most common extensions for MIDI files is .mid
Module files extend MIDI's concepts further, by providing embedded audio samples to ensure rendering that is as close as possible to the composer's intents.