What's the difference between Audio and MIDI?

Answer 1

MIDI is, simply put, a set of instructions you input into a track on your DAW (usually through a keyboard). These "instructions" are the notes you play on your MIDI controller that get recorded and then shown as MIDI row on your DAW.

These MIDI notes don't have any inherent audio/sound, they merely control whatever patch or plugin you load onto it. The same MIDI notes can control any instrument you load into the track, could be drums, violins, guitars, etc.

Audio is the actual sound wave. MIDI doesn't have any actual sound itself, it's the medium to get sound. You can record/commit your MIDI tracks once you are satisfied with how it sounds to convert it into audio. You can edit MIDI notes, you can't edit audio notes.

Answer 2

Assuming you're asking the question from a production standpoint:

Audio is any sound that is generated by any means and is heard aurally.

MIDI Musical Instrument Digital Interface is a technical standard that describes a communications protocol, digital interface, and electrical connectors that connect a wide variety of electronic musical instruments, computers, and related audio devices for playing, editing and recording music. It's the I and O's. Computer speak.

Now, once you've entered your MIDI by way of any number of thousand of midi capable devices it is turned into Audio so you can hear what you've created. You decide what the MIDI is going to trigger: a piano, a harmonica, a cricket.