• A file with the .MID or .MIDI file extension(pronounced as “mid-ee”) is a Musical Instrument Digital Interface file.
• Unlike regular audio files like MP3sor WAVs, these don’t contain actual audio data and are therefore much smaller in size. They instead explain what notes are played, when they’re played, and how long or loud each note should be.
• Files in this format are basically instructions that explain how the sound should be produced once attached to a playback device or loaded into a particular software programthat knows how to interpret the data.
• This makes MIDI files perfect for sharing musical information between similar applications and for transferring over low-bandwidthinternet connections. The small size also allows for storing on small devices like floppy disks, a common practice in early PC games.

Source: Lucent’s Computer