A MIDI “instrument” is known alternately as a sound module, a rackmount sampler, a rackmount synth, an external synth or an external sampler. It is simply an electronic device that can make certain kinds of noises (“you could be kind and call it music” – Mick Jagger). Unlike a traditional musical instrument, though, it doesn’t include a feature than would allow a musician to play it – it’s kind of like a piano without keys. It is designed to be played not by a musician directly, but by another device that the musician directly controls. The reason for doing it this way is so that a musician can use one controller (such as a MIDI keyboard) to play several different MIDI instruments at once by controlling them all simultaneously – when he hits a key on his MIDI keyboard, several different MIDI instruments can sound off at once if he has set it up that way in advance.
What can a MIDI instrument play? It can play whatever sounds have been built in to it – typically the sounds of a variety of different traditional instruments such as a flute or a guitar, plus a wide assortment of synthesized sounds. Each type of sound is known as a “patch”. Modern MIDI instruments are multi-timbral, meaning that they can receive MIDI signals on all 16 MIDI channels at once (or any combination of them), and play 16 different patches at once.
MIDI channels allow an electro-musician to “play” different musical sequences on different MIDI instruments from his MIDI keyboard – without this feature he could play many different instruments at once, but they would all play the same notes in the same sequence, giving a nice chorus effect but denying the musician the ability to build compositions composed of complex sequences. MIDI channels give the musician independent control of several different instruments at once from a single MIDI keyboard (or other MID controller). It’s as if he had 10 arms and 12 legs, all with just as good coordination as his original limbs.