A Wiki or wiki is a web application that allows users to add content, as on an Internet forum, but also allows anyone to edit the content.
Wiki also refers to the collaborative software used to create such a website (see Wiki software).
Since Wiki’s are often used these days as defacto CMS’s/knowledge bases these wiki comparison pages may be of use for someone deciding which wiki to use:
Wiki Engine Comparison – features of other wiki’s compared to Docuwiki
General and technical information for a number of wiki packages – wikipedia
Which Open Source Wiki Works For You?