What is the difference between a Major and a Minor third ?
The answer is - a semitone.
The main chords are made up of Root , Third and Fifth.
A Major chord comprises of Root , major Third and Fifth - C Major = C + E + G
A Minor chord comprises of Root , minor Third and Fifth - C Minor = C + Eb + G
What is the difference between a Major Seventh and a Seventh ?
The answer again is - a semitone.
The Seventh note of a major scale is one semitone below the octave note [octave = 8]
i.e. in the scale of C Major the seventh note is B and the eighth note is C.
B is the Major Seventh of C .
Cmajor7 chord = C - E - G - B
A seventh should really be called a flattened seventh because it is actually a whole tone below the octave note .
i.e. in C the seventh is Bb.
Bb is the Seventh of C
C7 chord = C - E - G - Bb
The 'Major' part of the chord name C Major 7 refers to the seventh and not to whether the chord is major or minor.
So you can have a Minor chord with a major seventh note added which would be written , for example , Am(maj7)
So what's a Minor Seventh then ?
The 'Minor' part of the chord name A Minor 7 does refer to the chord and not the note.
A minor seventh chord is a Minor triad with the Seventh note added [i.e. a whole tone below the octave]
Cm7 = Root + Minor third + Fifth + flattened Seventh
= C + Eb + G + Bb
Give your brain a bit of a rest then come back and read
this again - the concepts here are fairly crucial to theory and I know from
experience of teaching that they can be very confusing and may take a while
to sink in.
Also play the chords and look at the difference on the fretboard between a major and a minor chord , or a seventh and a major seventh chord .