According to Wikipedia "a wiki is a type of website that allows visitors to easily add, remove, or 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".
You can search Wikipedia through the link here above.
On the success of Wikipedia see links on this site's blog.
We are planning to add a free wiki software to our site to encourage cooperation among historians. For now you can freely use Writely