MIDI supports 128 program change messages over each of its sixteen channels. An oversight in the original MIDI specification was the lack of agreement about how different instrument sounds should be mapped to particular program change numbers. This meant that the same piece of MIDI music would sound totally different, depending on what MIDI instruments were used to play it back. General MIDI defines a list of 128 instrument sounds, and their program change numbers. General MIDI is a widely adopted feature in most MIDI instruments and computer-based MIDI products.
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.