The third spans three diatonic scale degrees, the minor third contains 3 semitones, the major third contains 4 semitones. The pitch ratio of the minor third is 6/5 or 7/6 and of the major third is 5/4.
Chords are formed as succession of thirds. When the interval between the tonic (first degree) and the mediant (third degree) of a chord is a major third the chord is a major chord.
When the interval between the tonic (first degree) and the mediant (third degree) of a chord is a minor third the chord is a minor chord.
The point is to memorize how it sounds. Some examples:
Major thirds from Bernstein's West Side Story:
The riff of Smoke On the Water begins with a minor third:
A descending minor third from Deep Purple's Mistreated:
The diminished arpeggio is a succession of minor thirds:
Beethoven's Fifth opens with a descending major third followed by a descending minor third:
The thirds are often played as double-stops like in Paganini's 24th capriccio:
Learn to hear the difference between major and minor thirds and between major and minor chords. It's one of the most important things in relative hearing.