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, 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 how to get started.