A Wiki is a Web site meant to be edited by any passing Web user. Wiki Web sites allow normal Web users to modify the pages they see. The clever thing about Wikis is not that anyone can edit them - although this is what makes them very useful - the clever bit is that you can edit Web pages while browsing, using your Web browser, without having to learn a complex language first.
To find out how to do this, try clicking the `edit' button at the bottom of this page. You'll see an editing window appear. You can type in this window, and change and add anything you like.
There is a simple but non-standardized text markup language used by Wikis, often called the formatting rules. This text markup language means you can apply simple text formatting to what you write. For example, you're using a Web browser to edit text and you might want to have some text appear in italic. You can't do it by selecting the italic menu item like you would in a word processor. What you do is you type it in surround by two pairs of single quotes, which makes the text look like this.
You can find partial details of the formatting rules for Alpha's Wiki here: http://alphatcl.sourceforge.net/wikit/6.html
If you're a hacker who already knows something about Wikis and what's behind the scenes in the World Wide Web, the following might be of interest.
From the Tcl'ers Wiki: 
What's a Wiki? see above
Wiki Wiki is the Hawai'ian term for "quick". Ward Cunningham is the one who coined the phrase of "Wiki Wiki Webs". The idea is that you edit pages in normal text mode, with a simple way to add new pages & hyperlinks between them.
It all works via CGI on a web server, so anyone with a web browser anywhere in the world can browse, follow links, and edit these pages.
For a few links to Wiki stuff, look at http://c2.com/cgi/wiki?WikiWikiWeb
A list of implementations of Wiki webs is at http://c2.com/cgi/wiki?WikiWikiClones. However, CL has trouble with these resources, unable, for example, to find MoinMoin  or ZWiki  there.
There is an internal project at Xerox Parc called "Sparrow" , which adds a fascinating new dimension to Wiki, by allowing users to edit portions of a standard HTML page:
SlashDot has had a similar concept for some time with Everything , but implemented as a sort of learning network, with the "strength" of links dependent on how often they're traversed.
"Wiki" can be understood in at least four senses: