"A wiki is a software platform for contributing, sharing, and
managing content. Any group that has a need for a collaborative
user driven content environment can benefit from using a wiki.
Perhaps the most prominent example on the Web of a wiki in heavily
active, popular usage today is Wikipedia, which is, as you might
expect, a wiki-based encyclopedia. The Wikimedia Foundation uses as
the basis of Wikipedia a GPL-licensed application called MediaWiki,
and so can you.
"From a user perspective, MediaWiki provides a simple, feature
rich environment for editing and contributing content. Content is
handled with a simple WYSIWYG editor that makes editing accessible
to anyone by default even without a login..."