Wiki is a software tool that allows collaborative writing. The biggest project that represents the power of collaborative writing is Wikipedia The founder of Wikimedia foundations Jimmy Wales gave a talk at TED and provided the details on how the collaborative writing is done using free open source media wiki software and how they resolve the conflicts that arise around controversial topic. He emphasized how with the help of few staff members and large number of volunteers the large volume of writing was accomplished in creating Wikipedia.
Use of wikis in political process by barrack Obama to organize the local activities.
The wiki markup is different then the html page markup. But now days wiki websites like wetpaint provide a nice rich text based interface to create wikis. They also provide the access control on how the wiki pages will be viewed and edited by its users. The front page of their website comes with a nice video explanation of how to create your on wiki pages.