What is a Wiki?

Wikipedia is a wiki, meaning it is a web application that allows a collaboration of users to edit, add, or delete content.  It is characteristically different than other content management systems (like a blog) because there is "no defined owner or leader, and wikis have little implicit structure, allowing structure to emerge according to the needs of the users."

Rather than being relatively linear as are many websites, sites that use wikis are fluid and interwoven. They promote meaningful associations between different related topic pages by allowing users to easily create multiple links.  The wiki is not a carefully designed and published page - instead it relies on the input of multiple users to create and polish its content.  The crafting of a wiki is an ongoing and collaborative process.

The first wiki was called "WikiWikiWeb" and was launched by Ward Cunningham on March 25, 1995.  He chose the word "wiki" as a substitute for the word "quick" after recollecting the quick shuttles that ran at the Honolulu airport (called the Wiki Wiki Shuttle).  The purpose of WikiWikiWeb was to "link together the experiences of multiple people to create a new literature to document programming patterns, and to harness people's natural desire to talk and tell stories with a technology."

Ward Cunningham

         Read more about wikis here.

Currently, the most popular wiki is Wikipedia.  However, there are many other sites that use wikis.

A list of some "notable" wikis can be found here:  Notable Wikis




References:
http://en.wikipedia.org/wiki/Wiki