Pre-requisite: This is an advanced explanation of musical concepts. If you don't have a basic understanding of Intervals,
Scales, Chord Construction, and basic Modes this might be like stepping off the curb and falling in a pit.
Please visit my web site (http://lessons.mikedodge.com) and read the Beginner To Advanced Series.
Those lessons explain much of the basis you should have before delving into these concepts.
For Diatonic Theory I will start with the term "Modes" and some concepts, and misconceptions, since a lot of people start with the patterns before they get to theory behind the Modes.