What is a Wiki?  

A wiki is a collaborative website which can be directly edited by anyone with access to it. Ward Cunningham, developer of the first wiki WikiWikiWeb, originally described it as “the simplest online database that could possibly work”. One of the best-known wikis is Wikipedia. WikiWikiWeb was the first site to be called a wiki. Ward Cunningham started developing WikiWikiWeb in 1994, and installed it on Internet domain c2.com on March 25, 1995. It was named by Cunningham, who remembered a Honolulu International Airport counter employee telling him to take the so-called “Wiki Wiki” Chance RT-52 shuttle bus linethat runs between the airport’s terminals. According to Cunningham, “I chose wiki-wiki as an alliterative substitute for ‘quick’ and thereby avoided naming this stuff quick-web.” Wiki Wiki is a reduplication of wiki, a Hawaiian-languageword for fast.

• A collection of websites of hypertext, each of them can be visited and edited by anyone. “Wiki wiki” means “rapidly” in the Hawaiian language.
• An online collaboration model and tool that allows any user to edit some content of webpage’s through a simple browser.
• A 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 pbwiki software)

Great resource to further your knowledge on Web 2.0 http://web2tutorial.wikispaces.com/