If you've ever seen a player piano in operation, you may remember a scrolling sheet of heavy paper with a series of short and long holes cut into it. The position of each hole represents a particular musical pitch while its length sets the duration of that particular note. By changing the rate at which scroll is fed through the piano mechanism, the tempo of the resulting music can be changed. Finally, while all the notes on a player piano are triggered with the same intensity, MIDI note messages include information about how hard each individual note is played. If you expect a particular part to be playing but hear only silence, it is probably because the notes are being transmitted over the MIDI wrong channel, or the receiving device is listening to the wrong MIDI channel.
Different instruments create different sounds.
So, while an acoustic guitar is capable of an enormous number of tonal shadings, even a virtuoso guitarist won't be able to sound like a trumpet. On the other hand, electronic sound-generators can readily create a wide variety of sounds. For example a MIDI keyboard might contain the sounds of a piano, a harpsichord, a marimba and a xylophone. This would make it possible to have a piece of music that began with the sound of the marimba, which then switched to a piano sound. MIDI supports 128 program change messages over each of its sixteen channels.