It's not really complicated, if you understand how scales are derived.
There are seven modes, each tied to one of the seven notes in a scale. You are probably familiar with two of them, Ionian and Aeolean modes, otherwise known as the major and minor scales, respecively.
Here's how it works. We'll use the key of C, as it's the simplist to work with. This is also much easier to visualize if you have a keyboard in front of you.
So, lets start with Ionian mode, or the major scale.
Here it is in C
C D E F G A B C
A major scale is made up of the following intervals.
Root, whole, whole, half, whole, whole, whole, half.
Now, if you were to play that C major scale again, except start on D and play it all the way through to D, you would be playing in Dorian mode. The interval pattern for that mode would then be.
Root whole half whole whole whole half whole
If you were to play the C major scale, but star on the E, you would be playing in Phrygian mode. The interval pattern for that mode would be.
Root half whole whole whole half whole whole
And on up the scale.
Starting on the IV of a major (F in C major) scale puts you in Lydian mode.
Starting on the V (G in C major) of a major scale puts you in Myxolidian mode, which sounds great over dominant 7 chords.
Starting on the VI note of any major scale (A in the case of C major) ends up being that major scale's "natural minor" scale. In the case of C major, the natural minor is A.
Starting on the VII of a major scale puts you in Locrian mode, but only whacked out jazz guys go there. <img src="images/smiley_icons/icon_smile.gif" border=0 alt="Smile">
Scales are the underpinning of everything. Modes are another example of that.
Hope that helped!
Someday we'll win this thing...