The Complete Idiot''s Guide to Music Theory

Part 1:
Tones
In the Mode
If a scale is a combination of eight successive notes (in alphabetical order, of
course), do any eight notes make a scale?
Not necessarily.
Once you get past the major and minor scales, all the other eight-note combi-
nations aren’t technically called scales; they’re called 
modes.
38
Modes date all the way back to the ancient Greeks, and the findings of Pythagoras
and Aristotle. In fact, it was Aristotle’s student, Aristoxenus, who formalized the
Greek scheme of modes, which included the Dorian, Ionian, Lydian, and Phrygian.
The name of each mode was based on the final note of the mode.
The number and use of modes were expanded in the era of the medieval church,
where they were called 
church modes
and used in the form of plainsong called
Gregorian chant. The last discovered mode, Locrian, is actually a theoretical
mode; it was never used in the same context as the other church modes.
Chronologically, modes were around long before scales. The major and minor
scales we use today came after the introduction of the various modes, and were,
in fact, based on the Ionian and Aeolian modes, respectively.
Note
There are seven essential modes, each of which can be thought of as starting on
a different degree of the major scale. You stay within the relative major scale;
you just start on different notes.
For example, the Dorian mode starts on the second degree of the major scale.
In relation to the C Major scale, the Dorian mode starts on D, and continues
upward (D, E, F, G, A, B, C, D). The same holds true for the Phrygian mode,
which starts on the third degree of the related major scale—in C Major: E, F,
G, A, B, C, D, E.
Modes are important when you’re constructing melodies. When you create a
melody based on a specific mode, you get to create a different sound or feel while
staying within the notes of a traditional major scale. You just start and stop in
different places. (Melodies based around specific modes are called 
modal
melodies.)
Ionian
If you’re a musician, you play the Ionian mode all the time without really
knowing it. That’s because the Ionian mode starts on the tonic of the related
major scale—and contains the exact same notes as the major scale.
The following table details the half steps between the notes of the Ionian mode.
While it’s convenient to
think of modes in relation
to a specific major scale,
modes are arrangements
of intervals in and of them-
selves. In practice, any
mode can start on any
note.