Wiki is a concept, more than a thing. It is a way of creating document pages using plain text markup and simplified hyperlinking.
By typing a name in MixedCase (also known as CamelCase), a hyperlink is automatically created to the document MixedCase. Pressing return on that name will create the file if it doesn't exist, or visit it if it does.
The markup used by Emacs-Wiki is intended to be very friendly to people familiar with Emacs. Type C-h v emacs-wiki-publishing-markup after this mode is loaded for more information on how to get started.
Wiki's are often associated with sites that allow collaborative editing of a website. emacs-wiki is not meant to produce this sort of site, although you can use Emacs to serve web pages. See emacs-wiki-httpd.el for more information. Note that this feature is not well-tested.