Chances are you have been using wikis and did not know it. The most famous wiki is called Wikipedia, a massive online encyclopedia. Below is an excerpt from Marshall Brain, How Wikis Work, which provides an excellent overview.

"Wikis are growing because, at their core, they are about as simple as can be. That simplicity means that people find them easy to use, just like e-mail and blogs....A wiki allows a group of people to enter and communally edit bits of text. These bits of text can be viewed and edited by anyone who visits the wiki. That's­ it. What it means is that, when you come to a wiki, you are able to read what the wiki's community has written. By clicking an "edit" button on an article, you are able to edit the article's text. You can add or change anything you like in the article you are reading...Virtually any topic with any sort of active community can, in theory, support a wiki. A wiki gives the community a way to gather information together and modify it as things change..Virtually any topic with any sort of active community can, in theory, support a wiki." Marshall Brain