Difference between audio and MIDI.
An audio cable conducts electrical signals that are "exact copies" of the sound waves produced by human voice or a musical instrument.
A MIDI cable conducts "text messages" which tell an electronic musical instrument or a computer, which notes should be played and what characteristics should these notes have. How faithfully these notes will be reproduced does not depend on the message itself, it depends on the instrument or the computer's ability to generate musical sounds.
Audio. Sound travels through the air by way of fluctuations in atmospheric pressure known as "sound waves". "Exact copies" of these fluctuations can be sent through a wire in the form of electrical waves. In that case we say that an audio signal is traveling through the wire. An audio signal carries a lot of information. It is this information which makes it possible to distinguish the particular nuances of a musical note when sung by two different singers or played by two different violins. This is why an audio signal demands a lot of space in order to be recorded.
The gurus of technology soon realized that there was a need to reduce the size of audio files, in these files were to be sent through the internet or stored in large quantities in memory sticks or iPods. This motivated the invention of compressed audio formats like mp3 and other formats. An mp3 file is a compressed audio file which suppresses some sounds we humans usually don't pay much attention to. It takes a rather well educated ear to notice the difference between an ordinary audio file and its compressed version.
MIDI. The acronym MIDI stands for Musical Instrument Digital Interface, which in simple terms means a protocol -a set of rules and agreements- that allow communication between electronic musical instruments of different manufacturers and computers. A MIDi signal consist of a set of text messages called MIDI messages. Each message asks its recipient instrument to perform certain musical action like playing a note of certain pitch, duration, volume and timbre. The message does not describe the shape of the sound wave that represents the note. It is the recipient instrument the one which has to generate that wave (the sound). Using a comparison, MIDI is like asking a chef to cook a lasagna. Audio is having to produce (cook) the lasagna. Because a MIDI file consists of text only, it is several hundreds times smaller than a mp3 file. Asking how good is the sound quality of the different instruments in a MIDI file is like asking how good a lasagna will be when ordered to a chef. It surely will depend on how good is the chef, in our example, the instrument that processes the messages.
Electronic instruments with MIDI interface.
When an instrument (a keyboard or a guitar, for example) has MIDI interface, it means the instrument can receive and send MIDI messages. When the instrument receives a MIDI message it can interpret the message and produce the requested notes. One wonderful thing about a MIDI instrument is that you can plug it to a computer or to other MIDI instruments. When connected to a computer it is possible use the instrument to feed notes into music programs like sequencers and music notation programs. Modern electronic instruments with MIDI interface can be connected to a computer using an USB port. Less modern instruments di not have a USB port. These instruments had two MIDI ports calles MIDI IN and MIDI OUT ports. In order to connect these ports to a computer it was necesarry to install a< MIDI card in the computer and use special MIDI cables (5 pin cables). There are many instruments still in use which work with MIDI ports and MIDI cables. If you are thinking of purchasing a MIDI instrument, make sure it is equipped with an USB port.
Most computers can process and play MIDI files, although usually the sound isn't very good. Motherboards or even many sound cards do not make a great effort to faithfully imitate the sound of different musical instruments.