The difference between a major chord and a minor chord is that in a minor chord, you flat the third.

Major and minor chords are made up of 3 notes. Major chords are made up of the root note (the tonic), 3rd, and 5th. So, an A major chord is made up of A, C#, and E. If you look at the A major chord shape, you will see it is made up of only these notes.

To make it a minor chord, you take the third (C# in this case) and flat it. In terms of the guitar, this means the note is played a fret lower. So rather than fretting the B string on the second fret, it is fretted on the first fret, giving you a C note.

So an A minor chord is A, C, and E.

As far as tonality goes, minor chords have a more somber sound whereas major chords are more "happy" and uplifting sounding.

Hope that helps!