Wikipedia (pronounced /?w?k?'pi?di.?/ WIK-i-PEE-dee-?) is a multilingual, web-based, free-content encyclopedia project based on an openly-editable model. The name "Wikipedia" is a portmanteau of the words wiki (a technology for creating collaborative websites, from the Hawaiian word wiki, meaning "quick") and encyclopedia. Wikipedia's articles provide links to guide the user to related pages with additional information.
Wikipedia is written collaboratively by largely anonymous Internet users who write without pay. Anyone with Internet access can write and make changes to Wikipedia articles. Users can contribute anonymously, under a pseudonym, or with their real identity, if they choose, though the latter is discouraged for safety reasons. The Wikipedia community has developed many policies and guidelines to improve the encyclopedia, however, it is not a formal requirement to be familiar with them before contributing. Since its creation in 2001, Wikipedia has grown rapidly into one of the largest reference web sites, attracting nearly 68 million visitors monthly as of January 2010. There are more than 91,000 active contributors working on more than 15,000,000 articles in more than 270 languages. As of today, there are 3,239,964 articles in English. Every day, hundreds of thousands of visitors from around the world collectively make tens of thousands of edits and create thousands of new articles to augment the knowledge held by the Wikipedia encyclopedia. (See also: Wikipedia:Statistics.)
Every contribution may be reviewed or changed. The expertise or qualifications of the user is usually not considered. This is possible since Wikipedia's intent is to cover existing knowledge which is verifiable from other sources, original research and ideas are therefore excluded. People of all ages and cultural and social backgrounds can write Wikipedia articles as most of the articles can be edited by anyone with access to the Internet simply by clicking the edit this page link (found at the top of every editable page). Anyone is welcome to add information, cross-references, or citations, as long as they do so within Wikipedia's editing policies and to an appropriate standard. Substandard or disputed information is subject to removal. Users need not worry about accidentally damaging Wikipedia when adding or improving information, as other editors are always around to advise or correct obvious errors, and Wikipedia's software is carefully designed to allow easy reversal of editorial mistakes.
Because Wikipedia is a massive live collaboration, it differs from a paper-based reference source in important ways. In particular, older articles tend to be more comprehensive and balanced, while newer articles more frequently contain significant misinformation, unencyclopedic content, or vandalism. Users need to be aware of this to obtain valid information and avoid misinformation that has been recently added and not yet removed (see Researching with Wikipedia for more details). However, unlike a paper reference source, Wikipedia is continually updated, with the creation or updating of articles on historic events within hours, minutes, or even seconds, rather than months or years for printed encyclopedias.