MIDI stands for Musical Instrument Digital Interface.
MIDI music is the computer equivalent of writing down music on paper and getting the computer to perform the piece of music each time it needs to be played. This means that the way it sounds depends on the computer's sound card, so the music will often sound different on different computer. We may also say that MIDI is very much like a player piano roll that specify the action of the piano, while the tone or effect is generated by the piano itself of different brand.
While WAV music sounds exactly the same on every computer. WAV file is much larger in size than the original MIDI.