Modes and Pentachord.
Well before we would talk about major or minor keys, scales and so on, music was "written" or considered as a modal system. Modes are named after the ancient Greek modes. Originally there were four of them. D , E , F , and G mode. They have been used until the 16th - 17th century in Church music and Gregorian music. D mode was the main one and was called the mode of god. Now, there are seven of them, one for each note of the Greek mode system.
Here is below, in ancient Greece, the idea behind the Pentachord and Modes. Penta (from Greek "five of something" and chord (chordon - "string"). The picture below, with the black dots, "explains" the principle of an instrument with five strings. Three full tones and one semitone. See the position of this semitone in D mode for instance and compered it with the minor key we have today. Take a look now at the picture: All transposed modes in C major and see the difference between D Dorian and D natural minor. This why we consider Modes as being different as Scales.
Now, let’s have a look at the picture: Modes in C major, below. They are all the modes in C major: C Ionian, D Dorian, E Phrygian, F Lydian, G Mixolydian, A Aeolian, B Locrian. Very often, one takes the image of the white keys of a piano played from C to C, to D to D and so on. (See picture below)The whites keys from C to C give the Ionian mode, from D to D the Dorian mode, from G to G the Mixolydian mode and so on. It is a matter of shift. See more in detail the Comparison Mode picture Below. You will notice the right shift among the notes. So, what is crucial here to understand is the result of that shift. In C Ionian, we have a major Third, (C, E, G. However, in D Dorian it a minor Third (F, A, C). This is the "purpose" of the modes. It is to give to each mode its colouration.