A chord is the layering of several tones played simultaneously - usually built on superposed thirds. Chords are defined by their root note and their quality (major, minor, 7, etc)and eventually by their inversion.
Triads
A triad is a chord made of three notes. It consists of a root and the third and fifth above it.
The most common triads are:
Major - has a major third and a perfect fifth
Minor - has a minor third and a perfect fifth
Diminished - has a minor third and a diminished fifth
Augmented - has a major third and an augmented fifth
On the staff below, the C major, C minor, C diminished and C augmented are transcribed:
triads
Seventh chords
These chords are triads with a supplementary diatonic third superposed. The most common seventh chords are:
Dominant 7 is a major triad with a minor 7 added.
Minor 7 - is a minor triad with a minor 7 added.
Diminished 7 - is a diminished triad with a diminished 7 added.
Half diminished - is a diminished triad with a minor 7 added.