Wikis (the term comes from the word "quick" in Hawaiian) are collaboratively edited hypertext publications directly accessible by a web browser. They can contain multiple pages dedicated to different subjects. Wikipedia is by far the most famous wiki on the Internet, but it is not the first one historically. The first wiki software was the WikiWikiWeb (now archived), created by Ward Cunningham back in 1995.
Wikis differ from the traditional content management systems due to the fact that pages have no specific owner, and in most cases, everybody is free to create new pages and to edit existing pages.