If we take the C major scale we find that the notes are arranged like this:
C D E F G A B C
C D E F G A B C
We can break that scale down to a series of half and whole steps like this:
C - D is a whole step, also called a whole tone
D - E is a whole step,
E - F is a half step, also called a semitone
F - G is a whole step
G - A is a whole step
A - B is a whole step
B - C is a half step.
By looking at what makes up a scale, we should be able to see a pattern emerge. In the case of this major scale, we see this pattern: Whole step, Whole step, Half step, Whole step, Whole step, Whole step, Half step. Now here is where the fun comes in. The different modes that we use are just this pattern beginning and ending at a different point. Let me illustrate.
Here is the pattern for two octaves of a major scale, also known as the Ionian mode: (W stands for Whole step and H stands for Half step)
W W H W W W H - W W H W W W
If we begin this seven part pattern on the second degree we get something that goes like this:
W H W W W H W (starting on the second W of the above pattern)
This is called the Dorian mode. This is a very neat minor sounding mode to play in. Think Halo splash screen...
If we start on the first H of the pattern we get this:
H W W W H W W
This is the phrygian mode.
The Lydian mode starts on the next degree:
W W W H W W H This one sounds a little like a major on the bottom end of the scale and a little minor on the top
W W W H W W H This one sounds a little like a major on the bottom end of the scale and a little minor on the top
The Mixolydian mode begins like a major scale and is major all almost the way through.
W W H W W H W
The Aeolian mode uses this pattern:
W H W W H W W This one will sound familiar to most guitar players since it is also known as the natural minor scale and the majority of rock and blues is built on this mode. The seventh degree of this scale is often raised in order to strengthen the pull back to the tonic pitch.
This last pattern is an interesting one. Because it has two naturally occurring tritones in it, many music theorists consider it to not be a mode at all, however it is almost indispensable for a jazz player. Here is the Locrian mode:
H W W H W W W
An Irish guy (and one of the better human beings around) told me a little saying to remember the order of the modes and it goes like this:
I Don't Play Like Most Average Lads
Ionian Dorian Phrygian Lydian Mixolydian Aeolian Lochrian
And that's that! Good Luck!