A wiki is a website that can be edited quickly and easily.
A person can make changes to a site at a touch of button. It is also very easy for people to comment on articles on a website.
This makes it ideal for collaborative authoring projects, such as a game design document, as it allows the full game design team to be more actively involved in the creation of a game design document.
A wiki design document is much easier to read than a traditional Microsoft Word created design document for several reasons:
1) The content can be broken down in to small segments that are less intimidating to read than a Microsoft Word page.
2) The links can be made between sections of content. This makes it much easier to navigate and jump between sections of a game design document.
3) As the viewer can leave comments, s/he is more actively involved. This makes it more likely that s/he will actually read the document.
4) A search engine can be used to quickly find key sections.