I transfered the Airtel Jingle from Sesh's phone to mine and when I played it I was disappointed. The tune didn't sound as melodious as it did on Sesh's Phone. Earlier I had got some vague idea that Midi files might be something that plays depending on the player app, this tune confirmed it.
So whats there in a Midi file?
Simple googling gave me lots of links describing the format. The general idea is that Midi file has a XML like approach. There are tracks which contains chunks. These chunks can be thought of as describing a node of music data. Where a node contains information like which instrument is to be played for how much time etc.
This makes the format very flexible. A player(sequencer) can ignore some nodes(chunks) if it doesn't know how to play them. Thus any sequencer if written properly will not crash even if a newer version of Midi file is played in it.
A very good app name kmidi which came with FC1 shows you clearly which particular instrument is currently being active. Do try that in case you are interested in understanding the usefulness of Midi File Format.
A very good reference is Midi format spec. As a side note, my Motorola A768i plays midi files with very less musically compared to a Sony Erricson or Nokia phone.