MIDI is a pretty neat protocol, involving a "language" that was established in the early 80's. The term does not reflect a certain piece of gear, nor does it supersede any other method of communicating music information with another type of protocol. Many times, there is a desire to control another keyboard or "sound module" (No keys on those!), and rather than attempt to play both units simultaneously, the primary controller(Master) sends a data signal in midi format to
the other unit.(The slave) Several slaves can be controlled by the Master, using the midi "thru" connectors for the units in between said Master and the last slave in what is called a "daisy chain" configuration. Midi protocol uses 16 discrete channels to be able to differentiate between instruments, voices, and even lighting controls. Some equipment recently implemented 32 channels, using an "A" or "B" designation to tell which group of 16 channels is to go to A or B, respectively.
Per each channel, there is data that acts like an "adjustment" for the following; Note 'on', note 'off'
'aftertouch', velocity, pitchbend, and volume, just to name a few. The range of "adjustment" is 127 steps, as far as the value range is concerned.
As instruments go, there is a plethora of sounds available; Some are vintage and generated by some form of synthesis, others are sampled, and therefore, a much more realistic rendering of a desired sound is achieved.
Many "complete" studios now include a midi keyboard of some sorts in the recording booth. This can be used for reference for desired sounds, or even used in the track mixing portion of a groove that is being "built up". You can also use a midi controller to play percussion. (Hey those are sounds too!)
Hope this starts you out with a better understanding....Accordianman