A wiki is a website collection of articles which can be edited by anybody, and a complete record of all edits is maintained forever. It's typically hosted on a website that allows visitors to add, edit or remove content in an easy manner. This facilitates collaborative efforts by many people, often simultaneously.
A wiki is considered a type of content management system. They differ from blogs or other systems in that the content is created without any defined owner or leader, and wikis have little implicit structure, allowing structure to emerge according to the needs of the users. They are particularly useful for taking notes in a structured way. The spirit behind a wiki is one where you feel free to jump in with both feet and "be bold".
Ward Cunningham said:
- Someone not familiar with authoring may have an idea, and the idea is a paragraph's worth of idea. They would write an editorial for a magazine, except a paragraph is too small. To write for a magazine, they would have to establish the context, say something important, say it in a way that a wide variety of people can understand it, and then bring it to a close. That's more than most people want to invest.
- But if you're reading somebody else's work, and you think, "Yeah, but there's another point," being able to drop in a paragraph that says, "Well, yeah, but there's actually this." There's an awful lot of counterpoint, the "Yeah, but..." kind of thought, on wiki. Discussion groups do the same thing, but with discussion groups it all gets lost. 
The first wiki created was "Ward's Wiki", named for its programmer Ward Cunningham. It focused on software engineering topics and is still in existence. Wikipedia was created using MediaWiki software, custom designed for encyclopedia authoring. From 10,000 articles and 500 users it has grown to be a top 10 website with over 1 million articles in English alone; each other language is a separate wiki, but with interwiki links. Related projects using the same software include Wiktionary and a huge image repository.
Some wikis are open to editing by the general public, but some have sign-up requirements. The New World Encyclopedia is the most serious academic fork of Wikipedia, financed by Rev. Moon's Universal Peace Federation. Its writers must submit two writing samples and their real-life academic credentials; they are paid for their submissions (either new articles or substantial revisions of Wikipedia articles). Citizendium is an all-volunteer project founded by Larry Sanger.
Conservapedia is a "wiki". Other notable examples are:
"Wiki" is from the Hawaiian word wiki wiki, meaning "quick." In part, the word refers to a simplified system of markup, which allows the most common kinds of HTML markup to be performed in a semi-intuitive way. For example, using the MediaWiki software, a user can italicize a word by preceding and following it with two apostrophes:
- ''italicized'' becomes: italicized
instead of using the HTML syntax
The technology was originally developed by Ward Cunningham, and saw early application among open source developers for maintaining code documentation. Wikimedia (the organization behind Wikipedia) developed the technology further, and made it available as open source software, under the name MediaWiki. MediaWiki is the software that hosts Wikipedia, Conservapedia, and many other wiki sites.