The Head is a separate unit that sits on top of the Quad box (4 big speakers in one box).
It contains all the controls like Volume, Treble, Mid Tones, Bass, Distortion / Gain, Reverb ect.
The more money you spend the better sound and more controls you will have.
I guess they are separate because it's is easier to carry and you can't fit the Head in the quad box cuz the 4 speakers have taken up all the room.