The design document is more of a design specification document that describes the application in its entirety. That means that all of the data-, architectural-, interface-, and component-level design is described for it. The following example showcases how the documentation would look. You can download a copy of the template from http://ec.europa.eu/idabc/servlets/Doc7e17.doc?id=18632:
For game design, the design document can be a bit more involved than it is for applications. A typical game design document will require the sections described to be filled out in as much detail as possible. You can download a copy of the template from https://docs.google.com/document/d/1-I08qX76DgSFyN1ByIGtPuqXh7bVKraHcNIA25tpAzE/edit:
Now, you might be thinking that this is a lot of work for applications or games. The key to success is never wrought from laziness or pure luck. In creating documentation that has as much depth as this, you are ensuring that you know exactly what you need to do and why you need to do it, and if you bring on team members, they will be able to read the document and fully understand your goals and intentions with little input from you.
This also means that you can keep yourself firmly in line with the project, forcing you not to add little pet features except as extra milestones after the project has reached the completion mark.