A wiki is a site that lets anybody who wants to edit any page on the site.
The concept was invented by Ward Cunningham, who created the 
Portland Pattern
Repository as an experiment in openness and simplicity in web design and
community building.
Wikis have become pretty popular over the years because they foster trust
and friendliness among all participants. Go ahead and visit Ward's wiki if you
want to understand the idea—I promise you'll be lost in there for weeks
and weeks. Or, check out Meatball Wiki
for in-depth musings on community, security, and why wiki
works.