its realy pretty simple - basic chords consist of 3 notes stacked on top of eachother in third intervals - since there are 2 types of thirds (major and minor) this leads to 4 possible combinations:

major third, minor third -> Major Chord
minor third, major third -> Minor Chord
minor third, minor third -> Diminished Chord
major third, major third -> Augmented Chord

example:

C major consists of C, E and G -> C to E is a major third, E to G is a minor third

C minor consists of C, D# and G -> C to D# is a minor third, D# to G is a major third.

its as simple as that.