I'm explaining with the help of the deluge what a seventh chord is, what different kinds there are, which ones occur in a major scale and what their possible inversions are.
Looking for for something in particular? Check the time stamps below:
0:00 Intro
0:35 triad recap
01:02 seventh chords
02:00 the different types of seventh chords
06:11 the seventh chords of a major scale
10:07 Inversions
15:15 omitting notes