A Demo Kit is a version of the Xbox 360 console used by game developers to demonstrate builds of games at conventions. They do not come with a Sidecar and thus include a standard 20GB Hard Drive. They were specifically designed to look like a Retail console to not draw attention from the public.
Demo Kits based on the Zephyr motherboard are white and come in two variants. Some kits have a white manufacturing label with a Demo sticker near the A/V port, while others have a black manufacturing label and no Demo sticker.