Wikis are websites that the users can edit directly within their browsers.
They are an excellent tool to share knowledge: It eliminates the hierarchy usually found in the online publication of knowledge, where there are clearly defined roles: people who generate the contents, people who put it on the website - and the users who just consume usually. With wiki websites everyone may take on any role (to some extent), the work may be shared around so people may more likely be able to do what they like more or what they are good at - Contents don't need to be passed through a chain of people till they are finally ready to be consumed, instead there can be a do-it-yourself way of doing stuff. So in the end by introducing all this sharing to the whole process is sped up, gets more efficient and just more fun...
Ward Cunningham, who has put the first wiki online, called it the concept the WikiWikiWeb after the hawaiian word for fast (wiki) because of the described gain of speed in the editorial process.
The contents of many of those websites get licensed under a free license (free content) to explicitly enable sharing of them. This makes wiki websites great sharing places for (free) knowledge. There exist many many wikis out there on the web - on many many topics. (If you haven't noticed yet: The Sharewiki itself is a wiki, of course - see the "edit" links? ...)