MIDI is a set of specifications for describing music. Instead of capturing and storing the sound of music, MIDI information describes the instruments, notes, and timing of the music, which can then be recreated on MIDI-capable devices.
A simple MIDI device might only reproduce the notes, while more sophisticated devices can create realistic-sounding music by synthesizing individual instruments. If multiple instruments can be recreated simultaneously, this is called polyphony (see Polyphonic Ringtones).
As an industry-standard, efficient file format for describing music, MIDI is often used for custom ringtones. Standard MIDI files can be transferred to a MIDI-capable phone and used as a ringtone.
MIDI files were commonly used as mobile phone ringtones before the support for the MP3/AAC standard was widely adopted.