The Musical Instrument Digital Interface (MIDI) has been since 1981 the prime communication standard that allows electronic musical instruments to send performance data between each other. Examples of such data include: what notes are being played (Note On/Off), the force a note being played at (also known as velocity), program changes, what tempo is being used for the current performance e.t.c. The protocol is at the heart of almost all digital keyboards (and even grand pianos), synthesizers, drum machines, Digital Audio Workstations as well as some non-musical stage equipment.
Before the protocol had been even established, there were several proprietary means of communication between instruments. Roland’s Digital Control Bus (DCB) allowed some synthesizers to transmit note information along with program/patch changes. However, this was only available on Roland synthesizers and there was no way to make an instrument from another brand to work with it. Given these problems, several manufacturers came together to develop what would eventually become MIDI. Since then, the MIDI standard has gotten several extensions such has General MIDI, GM2, MIDI time code and MIDI Polyphonic Expression.