MIDI messages are based on an 8 bit byte: MIDI values (note number, velocity, etc.) generally have a range from 0 -127.  Because it is a serial interface, MIDI can use a simple two conductor and ground  cable and an inexpensive 5 pin DIN connector. 

Originally devised in 1983 to have one synthesizer control another, its use has now expanded to include synchronization (MIDI Time Code), machine control (MIDI Show Control) and a myriad of device to device communication applications.