Wikitext language, or wiki markup, is a lightweight markup language used to write pages in wiki websites, such as Wikipedia, and is a simplified alternative/intermediate to HTML. Its ultimate purpose is to be converted by wiki software into HTML, which in turn is served to web browsers.
There is no commonly accepted standard wikitext language. The grammar, structure, justification, keywords and so on depend on the particular wiki software used on the particular website. For example, all wikitext markup languages have a simple way of hyperlinking to other pages within the site, but there are several different syntax conventions for these links. Many wikis, especially the earlier ones, used CamelCase to mark words that should be automatically linked. In MediaWiki, this convention was replaced with the [[…]] notation, which Wikipedia calls "free links".
"Wikipedia:Free links". Wikipedia, the Free Encyclopedia. http://en.wikipedia.org/wiki/Wikipedia:Free_links. Retrieved 2008-08-08.
Different Wiki programs may support use of different sets of HTML elements within wikitext. In some cases, permitted HTML elements may be configured by individual wiki sites. MediaWiki supports many common HTML tags.
Taken from http://en.wikipedia.org/wiki/Wiki_markup