What does it mean for a piece of music to have major harmony, minor harmony?

Harmony is based on scales which are constructed according to a certain set of intervals which create a specific sound thereby. For a piece of music to have major harmony, it must be based on a major scale, which is to say it has a major key signature. The term 'interval' expresses how many semitones apart one note is from another (a semitone being the pitch difference between a C and a C sharp, the smallest difference in sound in the Western musical tradition). In a major scale the pattern of intervals is: 2-2-1-2-2-2-1. Since two semitones make a tone, this can be said as two tones, semitone, three tones, semitone. This configuration of tones creates a sound that is upbeat, many would say it sounds 'happier' and lighter than a minor scale. This is because in a minor scale the configurations of intervals is modified to create a darker, sadder tonality. This would be delineated by having a minor key signature such as D minor. In D minor two things are majorly altered, there is a flattened third note so the usual F# is an F natural (down a semi tone), and the seventh note sounds raised which is created by making the pitch of the sixth note lower(lowering the B to a B flat, a semitone down). Major and minor harmonies are both diatonic harmonies; if a piece is in a major key it is diatonic since it sticks to the key signature. It is possible to have passages of music within a piece that are not part of the key. You can have minor passages in a major piece if at that particular time it is based upon minor chords.