Harmony is the use of different notes (pitches) simultaneously. Melodic music consists of notes played one after another while harmonic music consists of two or more notes played simultaneously.
Most harmony is based on chords. A chord is a group of notes played simultaneously (at the same time). The basic chord is named a triad. It consists of three notes: the root (any note you like) together with the third and the fifth above the root. When the notes of a musical chord are played separately, one after the other instead of together, we use the term arpeggio.
Chords can be majors and minors. The third above the root is major (two tones) in major chords, the third above the root is minor (one tone, one semitone) in minor chords. This is the reason why major chords sound bright and happy and minor chords sound sad and mournful.
Concord, or consonance, is a nice and pleasant sound that you get when some notes are played together, such as harmonic intervals of 3th, 6th, and 5th , 8th ones.
Discord, or dissonance, is an awful and unpleasant sound that you get when some notes are played together, such as harmonic intervals of 2nd and 7th ones.
