The vision is the first step in game design, and involves determining the basic principles of the game -- from things like the genre, to what will make it different to other games, and various rules that the game will be designed around -- but none of this should touch on how the principles will actually work.
This gets covered in the following design step, where documents are churned out covering all aspects of how everything will function and come together. These documents are looked over by different groups to check for design, technical, art and other issues that may arise when bringing a vision to fruition.
From there, it's on to the actual implementation of the design -- no easy task -- and then iteration until the product is where it needs to be. Have a read of Heretic's article to learn more about these steps in the design process, and to get a feel for what it takes to bring a game from an idea to the store shelf.
