Major and minor

Major chords sound bright and stable. They use a major third (4 semitones from the root). Example: C major = C, E, G.

Minor chords sound darker or sadder. They use a minor third (3 semitones from the root). Example: A minor = A, C, E.

Many songs use a mix of major and minor chords. The difference is only one semitone in the middle note, but it changes the whole mood of the chord.