A full Wiki can be added to an existing site design in order to allow the creation and maintenance of content either by yourself or by the community. Pages can contain text, graphics, discussion etc.
The number of registered users and pages available is limited only by the constraints of the hosting package chosen.
The Wiki can either be the home page of your site or an additional sub component.
The Wikipedia that we use is called MediaWiki and is the same Wiki that Wikipedia.org is based upon so the look and feel will be familiar to many users.