M.I.D.I. Primer
M.I.D.I. stands for Musical Instrument Digital Interface.  It is simply a series of messages that keyboards and synthesizers use to play notes or change the sound of a synthesizer.  A Virtual Instrument Track in Mixcraft contains Virtual Instrument Clips, which contain MIDI data.

MIDI Notes
When you play the middle C key on a music keyboard, it sends a short message that translates to 'play middle C'.  When you let go of the key, a message is sent that translate to 'stop playing middle c'.  In addition, the message contains other information, such as the MIDI channel and key velocity.  There are 16 MIDI channels which allow you to route MIDI messages.  The key velocity is a number from 1 to 127 which describes how fast the key was depressed.  A velocity of 1 would hardly be heard and a velocity of 127 would be full volume.

Each MIDI Note message contains the following information:  Note Value (C0 - C10), Key Velocity (1-127), MIDI Channel (1-16)

Note Value
This is the value of the note.  There are 128 possible MIDI notes.  Note 0 corresponds to 'C0', where 'C' means the note C and '0' means octave 0. Note 127 corresponds to G10.  A normal eight octave MIDI piano keyboard has a range from A1 to C9.

Key Velocity
This is how fast the key was pressed.  Faster velocities usually corresponds to louder sounds or may trigger extra sounds, depending on the synthesizer.

MIDI Channel
This is the channel that the note is on.  MIDI channels allow you to route specific channels to specific synthesizers and/or tracks.

MIDI Controllers
A controller is another type of MIDI message that controls other parameters.  The most commonly used controllers are Modulation and Pitch Wheel.  A standard MIDI piano keyboard has two wheels that correspond to modulation and pitch wheel (pitch bend).

Modulation
This is also known as vibrato and will usually cause the pitch to oscillate around the note.  However, it depends on the synthesizer being used.  Modulation has a value of 0 to 127 (127 is the most extreme modulation).

Pitch Wheel (Pitch Bend)
This controller usually bends the pitch up or down a half step in pitch.  However, it depends on the synthesizer being used.  Pitch wheel or pitch bend has a value from -8191 to 8192.

Other Controllers
There are other controllers that are useful for various synthesizers.  


Learn more about MIDI on Wikipedia