So far we've learned melodic intervals (melody) and harmonic intervals.  The next step is harmony, which is built upon harmonic intervals, using more than one harmonic interval creates a chord. When three notes are arranged one over another in a series of thirds, they create a triad. The first note of a triad (the bottom note in root position) is called the root.  The root is treated the same as a tonic of a scale.  It is the foundation of the chord.  The next note which is a third above the root, is called the third.  The next note which is a fifth above the root and a third above the third is called the fifth of the chord. 

Major and Minor Chords

Major and minor chords are two most common chords in Western music.  They are regarded as consonant, having a good sound, and are therefore widely used.  They are constructed as follows:

Major Chord - contains a major third (root - 3) - contains a perfect fifth (root - 5)

Minor Chord - contains a minor third (root - 3) - contains a perfect fifth (root - 5)

You will see the difference is in the third.  This difference is refered to as a triad quality.

Augmented and Diminished Chords

The augmented and diminished triads are considered to be dissonant, having a tense sound or "bad" sound.  They are constructed like so:

Augmented Chord - contains a major third (root - 3) - contains an augmented fifth (root - 5)

Diminished Chord - contains a minor third (root - 3) - contains a diminished fifth (root - 5)