From the Wikipedia article at http://en.wikipedia.org/wiki/Wiki
"A wiki is a type of website that allows the visitors themselves to easily add, remove and otherwise edit and change some available content, sometimes without the need for registration. This ease of interaction and operation makes a wiki an effective tool for collaborative authoring. The term wiki can also refer to the collaborative software itself (wiki engine) that facilitates the operation of such a website, or to certain specific wiki sites, including the computer science site (an original wiki), WikiWikiWeb, and the online encyclopedias such as Wikipedia.
Sam Rose distinguishes two types of wiki's, depending on whether community or content is primary:
"CommunityOverContent" wikis, which means there is some very cutting edge thinking and discussion going on, but the purpose of the wiki is for a group of close collaborators to create a PatternLanguage, as opposed to creating pages for a publication. Readability is less important than evolution of mutual understanding, in those wikis.
When Wikipedia was founded, and when Media Wiki software was created, the purpose of the wiki in that case was ContentOverCommunity, the community was dedicated to content. So, the discussion was moved off of the content page, and to a talk page, and the content pages in Wikipedia were organized for maximum readability.
Most communities that use wiki are usually some degree of a hybrid of these two purposes."