MIDI ringtone files are created with software that tell the cellphone what ringtone sound to play from inside the cell phone. MIDI is a standard format for polyphonic ringtones.
Many free ringtone sites offer a large selection of popular songs that have been "translated" into MIDI ringtones. They don't sound like the original songs on your favorite CDs, but the basic melody has been converted into a monophonic or polyphonic version. MIDI ringtones are popular because they often take up less of your valuable cell phone memory, but still sound enough like the songs that are currently popular to satisfy cell phone owners.