Terms such as ‘altered’ and ‘extended’ chords are relatively simple to explain. Extended chords add other notes from the scale to the existing root-3rd-5th triad (for example, C-E-G), so ...

All the chords built on other notes in the scale of C are minor, except chord VII - the notes B – D – F, which is diminished. When a seventh is added to a chord this is known as a seventh chord.