Wikipedia is the biggest (I think!) and most well known wiki, but there are lots of others. It's actually a piece of open-source server software that allows users to create and edit articles using any web browser, with hyperlinks to external sites and crosslinked pages. Wikis also allow the organsiation of articles to be created and altered in addition to the articles themselves, although most wiki sites, either public or private, have moderated rules governing this. Wikis encourage a free and democratic sharing of information on almost subject you can think of whilst making the sharing of this information highly accessible to non-technical users.
whilst the internal management of wiki sites varies, most being quite strict about content and format, others not so, big wikis like Wikipedia tend to operate by consensus - and this is, imo, the problem with Wikipedia; just because a majority of people agree that something is true and accurate, doesn't mean that it is. However on the whole Wikipedia doesn't do a bad job and it's accuracy is generally very high, although I've caught it out a few times (a terrible article on Piero della Francesca which I just couldn't allow to remain in the form i found it.) But that's what it's all about!
Although most wikis allow anyone to contribute and edit, it's generally best to read the rules and regs and see how other contributors do it, otherwise you'll find your article in all likelihood deleted or mercilessly edited. Most wikis have "Talk" pages where you can bounce your ideas around before contributing, and "sandboxes" where you can rough your article out before posting it.
Also if you just delete or rip chunks out of a wiki article without giving justifiable reasons why, or posting stuff without backing it up with independently-verifiable sources, you'll again probably find your actions undone by a moderator or other contributor and getting a warning, even if you're unregistered and anonymous, because they keep IP logs.
I made what i thought was a wonderful contribution to the Battlestar Wiki and it got ripped to shreds, not because of any factual errors on my part, but because they thought it was irrelevant and rambling