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 -
i.e. C Major = C + E + G
A Minor chord comprises of Root , Minor third and Fifth -
i.e. 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 thus the Major Seventh of C .

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 thus the Seventh of C

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


Exercise

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 .