MIDI is a series of instructions routed via an interface so that one musical device can control the behaviour of another. The Key elements of MIDI are as follows...
These are the sockets typically supported by MIDI hardware. MIDI devices can be connected via 5-pin cables or, more recently, USB connectors and even FireWire.
MIDI as blocks
The friendly face of MIDI, as seen in a typical sequencer's Key Editor window. Each block represents a note of a specific duration at a specific pitch.
MIDI as a list
Here's what MIDI looks like in detail: row upon row of assorted information, including how hard the notes were struck and to which channel they're assigned.
The sequencer is not merely a storage device. It can route the message to a sound source, such as a synthesizer, which will trigger a certain sound when the Note On information is received. And when you take your finger off the key, a Note Off message is sent to tell the synth to be quite.
MIDI events can also be entered into the sequencer manually, which is handy if you're a crappy keyboard player. just draw them in on screen, assign as instrument to the part, hit the play button and then listen to your creation over the speakers.
Bear in mind that sequences of MIDI data can be recorded as well, and played back simultaneously from, multiple sequencer tracks set to different channel numbers, each assigned to different instruments, and you'll have some idea of how cool things could get. MIDI track 10 could be firing the bass drum, snare and cymbals of a virtual drum kit, while track 1 plays a bass sound, track 2 triggers a synthesized string section and so on. You don't even need external sound generators to make such noises. Software-based instruments that plug into the sequencer are on hand to receive whatever MIDI data you throw at them, their output adjustable by means of a virtual audio mixer.