Game Design Document is the so called "Bible of game development project". It's a living design document you will fill and update during the development of your game project. It's a method of communication between the developers with various roles and tasks. Game Design Document is usually attached in a contract with a publisher.
You can have text, images, diagrams, concept art and other media. If you do your GDD in a wiki, you can also use links to refer to documents outside of the GDD. The form of the GDD has no industry standard. It can be a word document or a living document in a wiki for example. We strongly suggest you to use wiki, shared google document or anything else that is 'living' and easy for everyone to see and update!
