Some wikis support graphics, but it's safe to say that most primarily contain text. The original WikiWikiWeb was created by Ward Cunningham, but I've found that site's explanations overly abstract and disorganized. It's interesting to browse around in Ward's original wiki, though I'll warn you that it's easy to become brain-boggled in the process. Since the wiki software itself is open source, a vast number of wiki clones have arisen, written in a wide variety of languages and running on a wide variety of platforms. I used the WikiWeb site to collect questions for the annual Netters Dinner Survey at Macworld Expo this year.
Pages start in one of two ways - either essentially blank or relatively complete.
My goal is not to do all the work on the first pass myself, but to put enough material up there that others, when I send them the URL to that page, will be jogged into adding text, making changes, or perhaps just making comments. Plus, I can make changes any time I want without needing to distribute new versions. This approach can work well for starting a document no one desperately wants to create, since it spreads the workload. In this case, someone always ends up taking responsibility for the document, which entails copy editing, checking links, and tweaking the formatting. At other times it makes more sense to take a mostly completed document and put it up on a wiki page for review and editing.