A game design document (often abbreviated GDD) is a highly descriptive living design document of the game design for a video game. A GDD is created and edited by the developer team and it is primarily used in the video game industry to organize efforts within a developer team. The document is created by the developer team as result of collaboration between their designers, artists and programmers as a guiding vision which is used throughout the game development process. When a game is commissioned by a game publisher to the developer team, the document must be created by the developer team and it is often attached to the agreement between publisher and developer; the developer has to adhere to the GDD during game development process.
Now, exactly how to go about creating this document was something of a mystery, so I decided to exercise my google-fu and round up some resources on the subject. Rather than rewrite what these resources already say, I'll link them here for you!
- Brenda Brathwaite on her blog Applied Game Design has an excellent post discussing in detail the first section of the game design document. Since this is really the only section that's universal to all game types, it was very useful to see an in depth discussion of it.
- Mark Baldwin has created a template that covers most types of games. Obviously your game may not need some sections, and may need sections that aren't listed. Your mileage may vary.
- Intelligent Artifice has an old blog entry listing several game design documents for real games.
- The Gamasutra site has a pretty good article covering game design documents, though they are focusing more on people pitching their idea to a company rather than on independent developers who plan to self-publish.