GB's are wonderfully tortuous things (especially if you are a procrastinator) but also highly motivating and inspirational at the same time. As the others have mentioned, GBs are built around a theme, specific kit or range of kits, or broad time period depending on the desires of the facilitator and group. They usually have a definite start and end time and a badge is awarded by the facilitator to the participants depending. Ideally, it's a great way to share in a common build experience and leverage shared resources and methods along the way. It's excellent for raising the bar and pushing the envelope as well as providing a great opportunity to form relationships with fellow modellers, often scattered across the globe, in a way not possible through ordinary means.
Anyone can start a GB and, depending on the rules laid down, anyone can participate. It's pretty wide open as to how each GB is run and there's no set-in-stone rulebook for their conduct.