Grid beam is a modular and re-useable building method. Basically you can think of it as large meccano. It utilises long sticks of wood with multiples holes. These can be bolted together to form simple and complex structures. Joins are very strong as they use three sticks in a triangular structure which compress against each other to form an incredibly strong junction. Grid beam can be made in wood or metal. The cross section must be square and any wood used must be straight.
Some interesting websites (and pictures of hippys) can be found here: