A guitar amplifier head is used in conjunction with a speaker cabinet and is commonly known as a "half stack" - so called because the amplifier sits atop of the speaker cabinet.
Creating a Full or Half Stack
A full stack requires two speaker cabinets on top of each other with the amplifier head on top of the cabinets - this was a common configuration for rock guitarists in the 70s and 80s, particularly with those musicians who used Marshall amplifiers and cabinets.
Controlling Sound Output
The guitar amplifier head contains the electronic components and valves that create the sound and has all of the control interfaces for sound and frequency adjustment. It is connected to the speaker cabinet by a jack cable and some models have the ability to connect to several speaker cabinets although care and consideration must be given to the ohm rating of the speakers in order to achieve optimum volume.