Short for Musical Instrument Digital Interface, MIDI is a standard for digitally representing and transmitting sounds that was first developed in the 1980s. The MIDI sound is played back through the hardware device or computer either through a synthesized audio sound or a waveform stored on the hardware device or computer. The quality of how MIDI sounds when played back by the hardware device or computer depends upon that device's capability.
Many older computer sound cards have a MIDI port, as shown in the top-right picture. This port allows musical instrument devices to be connected to the computer, such as a MIDI keyboard or a synthesizer. Before connecting any MIDI devices to the computer, you need to purchase a cable that takes the MIDI/Game port connection into the standard 5-pin DIN midi connector or a USB to MIDI converter. If you do not have a MIDI port, the most common way today to connect a MIDI device to a computer is to use a USB to MIDI port cable.
The file extension .mid is used to save a MIDI file.