[Polo]

Imagine sheet music - it's just telling you what notes and how they are played, and needs a musician to make it into actual sound. This is how MIDI is, where the MIDI file is the sheet music, and the computer/software is the instrument, turning the notes from the file into the sound you hear.

Just as the musician can play the song using different instruments, MIDI can do this too, which is why MIDI files sometimes sound different when played on different computers (though because of General MIDI, they at least sound similar).

Hope that makes it clearer.