MODES

The usual description of modes is a group of scales related to the major scale.  While this is true, in practice at first it’s probably better to think of modes as a number of different sounding scales with unusual Greek names.  They are as follows:

IONIAN
DORIAN
PHRYGIAN
LYDIAN
MIXOLYDIAN
AEOLIAN
LOCRIAN

Each mode refers to a scale made up from the intervals of a major scale.  The intervals of a major scale (or Ionian) are:
Tone, Tone, Semi-tone, Tone, Tone, Tone, Semi-tone, or 2 Frets, 2 Frets, 1 Fret, 2 Frets, 2 Frets, 2 Frets, 1 Fret.

To make the Dorian mode the pattern is changed by starting at the second interval as follows:
Tone, Semi-tone, Tone, Tone, Tone, Semi-tone, Tone, or 2 Frets, 1 Fret, 2 Frets, 2 Frets, 2 Frets, 1 Fret, 2 Frets.

This method of starting at the next interval is used for each mode:

IONIAN

DORIAN

PHRYGIAN

LYDIAN

MIXOLYDIAN

AEOLIAN

LOCRIAN

T – T – S – T – T – T – S

T – S – T – T – T – S – T

S – T – T – T – S – T – T

T – T – T – S – T – T – S

T – T – S – T – T – S – T

T – S – T – T – S – T – T

S – T – T – S – T – T – T

In the same way as the Ionian scale is exactly the same (or another name for) the Major scale, so the Aeolian scale is exactly the same as the Natural Minor scale. It is this relationship between scales that leads to the major scale having a relative minor scale as demonstrated in the circle of 5ths. For example:
C Major and A minor.