The term MIDI stands for Musical Instrument Digital Interface. Do not confuse it with the term Midi as applied to Hi-Fi units, which is a description of their size (Midi as opposed to Biggi). MIDI is what is known as a serial connection. The data are sent as a series of long and short pulses in groups of eight. Additionally there is a pulse on either side known as 'start’ and 'stop’ bits. Just over 30 000 of these pulses can be sent every second – 3000 ‘groups' known as ‘bytes' (31.25 kbaud in tech-speak).
The connections are via Deutsche Industrie-Norm (DIN) plugs which, although physically identical to audio DIN plugs, are differently wired (Figure A2.1, top).