A wiki is a web site that provides a convenient syntax for users to edit pages. Many wikis, including all Wikia, allow any user to edit any page. However, it is also common to have wikis edited by small groups of persons.
Origin of wiki Edit
The wiki concept starts with the WikiWikiWeb, nominally the "Portland Pattern Repository". You can find it at Wiki:WelcomeVisitors. Anyone on the Internet can help author this site. The intention was to discuss "PeopleProjectsAndPatterns in SoftwareDevelopment" by letting users write and edit wiki pages. As the WikiWikiWeb grew, people established several SisterSites for other topics.
One of those sister sites was Meatball, which you find at Meatball:MeatballWiki. Meatball is about online communities and BarnRaising. Their wiki script, UseModWiki, became the basis for online reference site Wikipedia, which you find at Wikipedia:Main Page — an attempt to use wiki to build a free encyclopedia, distributed through the GNU Free Documentation License. They took many features from UseMod and put it in their wiki script, MediaWiki.
Wikipedia and MediaWiki advanced the wiki concept several ways. To make a link, WikiWikiWeb editors SmashWordsTogether to form a WikiWord. Wikipedia instead adopted the UseMod support for the FreeLink; you use two pairs of square brackets, such as [[social bookmarking]] for social bookmarking. This makes it easier to write wiki pages about arbitrary keywords.
Wikipedia also introduced the concept of neutral point of view or NPOV, an attempt to produce a useful reference from editors who disagree. However, NPOV introduces some serious restrictions on content. One result of this was Wikinfo, which you find at Wikinfo:Main Page, an encyclopedia and fork of Wikipedia which drops the NPOV restriction by switching to sympathetic point of view.
Wikia, which you find at w:Wikia, and of which this Internet wiki is a part, takes the MediaWiki engine from Wikipedia, and applies to a collection of hundreds of wikis. Wikia also has free links, and it also uses the GNU FDL copyleft.
Read-only and private wikis Edit
Some wikis are read-only, which allows one or a few users to easily build a web site. Some wikis are private, thus only the few editors themselves can see the result.