What's the difference between a major chord and a minor chord? What's the difference between a major key and a minor key?
The different is the third.
C major chord: C (root), E (third), G (fifth)
C minor chord: C (root), E♭ (third), G (fifth)
C major scale: C, D, E, F, G
C minor scale: C, D, E♭, F, G
C major has a major third -- E -- while C minor has a minor third -- E♭.
Remember that major literally means big, while minor means small. The distance from C to E is bigger by one semitone (one half-step) than the distance from C to E♭.
Now, what about the sixth and seventh notes of the scale? It's true that in standard C major those notes are A and B, while in standard C minor, they are A♭ and B♭. But, fundamentally, those notes don't hold any weight.
You can make A♭ and B♭ as flat as you want, and if the scale includes E, it's still going to sound major. Conversely, as long as you keep E♭, you can use A and B natural, and it will still sound minor.
The sixth and seventh notes have important roles to play, but it's the third that makes all the difference between major and minor.
Tomorrow, we'll take a fresh look at three-chord rock, in major and minor.