Wiki
A wiki is a website that allows the easy creation and editing of any number of interlinked web pages via a web browser using a simplified markup language or a WYSIWYG text editor. Wikis are typically powered by wiki software and are often used to create collaborative websites, to power community websites, for personal note taking, in corporate intranets, and in knowledge management systems.
-- Wikipedia
Wikis are generally have a 'page' as their base unit and usually have no or limited support for hierarchical navigation menus. Pages are navigated by following links in the text of other pages, by searching, and sometimes by browsing categories. Wikis are designed as collaboration tools with many people creating and editing pages. They store each version of a page so that the history of changes can be viewed and inappropriate edits can be reverted.
The lack of hierarchical menus in wikis can make them poor candidates for building websites that wish to have a very visible structure. This same property however, makes them excel at holding and navigating large numbers of loosely related pages on a wide range of topics. Because there are not fixed navigation menus, there is need to force pages to live in a particular category. Instead, pages can belong to many categories and linked-to from many pages, creating a rich web of information in which many paths can lead to the same places.