A wiki is a collaborative writing space that anyone can edit. You're no doubt familiar with the Wikipedia. What makes the Wikipedia interesting and problematic (from the standpoint of research papers, at least) is precisely the fact that it is a wiki.
But there is more to a wiki than the ability to quickly edit (and/or corrupt) the information hosted on them. Integral to most wikis is the ability to see a particular page's history and to comment on the page as it is being developed. This allows users to trace the evolution of a jointly-authored space; to revert to a previous version if something goes amiss; and to discuss whether the changes that are being made for the better. On pbwiki, you can "Add a comment" at the bottom of each page, and in the upper right of each page is the "page history."
The first wiki was imagined and implemented by Ward Cunningham, who wanted to find a way for software engineers work together on projects at different times. To describe the tool he developed, he chose the word Hawaiian word "wiki," which means "quick." Wikipedia is, of course, a very good source of information about wikis.