A Wiki is at its core a piece of web server software that allows users to freely edit and create content using a Web browser. It's pretty slick, allowing for anyone to edit posts and add their information/perspective quickly and easily. One of the most incredible examples of how a Wiki can be used to great effect is the Wikipedia project. Just go check it out to see what it's like when the whole world gets together to pool their knowledge resources! Nifty, eh?

Another fantastic Wiki is Everything. You can spend hours perusing Everything2 and Wikipedia! Good fun for brainiacs. :)

A Wiki can be a terrific game design & development tool if set up well. It also takes a commitment from the community it serves, since people really have to add to the thing to make it work. It dies off somewhat without participation.

We have a Wiki-like system in play here, in the form of the Game Dev Sourcebook. In this case, it's not using Wiki software, but the "book" system is similar in a lot of ways. Go check out the Sourcebook to see how we're (trying) to use the Wiki concept to develop a community-driven resource here at Game Dev School.