This is the official word from Wikipedia:
"This is an introduction to the Wikipedia project for visitors. We also have an encyclopedia article about it at Wikipedia. See also: list of overviews."
"Wikipedia (IPA: /ˌwikiˈpiːdi.ə/ or /ˌwɪkiˈpiːdi.ə/) is a multilingual, web-based, free content encyclopedia project. Wikipedia is written collaboratively by volunteers from all around the world. With rare exceptions, its articles can be edited by anyone with access to the Internet, simply by clicking the edit this page link. The name Wikipedia is a portmanteau of the words wiki (a type of collaborative website) and encyclopedia. Since its creation in 2001, Wikipedia has grown rapidly into one of the largest reference Web sites on the Internet.
In every article, links will guide you to associated articles, often with additional information. Anyone is welcome to add further information, cross-references, or citations, so long as they do so within Wikipedia's editing policies and to an appropriate standard. One need not fear accidentally damaging Wikipedia when adding or improving information, as other editors are always around to advise or correct obvious errors, if needed, and the Wikipedia encyclopedia software, known as MediaWiki, is carefully designed to allow easy reversal of editorial mistakes.
Because Wikipedia is an ongoing work to which in principle anybody can contribute, it differs from a paper-based reference source in some very important ways. In particular, older articles tend to be more comprehensive and balanced, while newer articles may still contain significant misinformation, unencyclopedic content, or vandalism. Users need to be aware of this in order to obtain valid information and avoid misinformation which has been recently added and not yet removed. (See Researching with Wikipedia for more details.) However, unlike a paper reference source, Wikipedia can be constantly updated, with articles on topical events being created or updated within minutes or hours, rather than months or years for printed encyclopedias.
Wikipedia is a registered trademark of the non-profit Wikimedia Foundation, which has created an entire family of free content projects. On all these projects, you are welcome to be bold and edit articles yourself, contributing knowledge as you see fit in a collaborative way.
If you have not done so, we invite you to take a few moments to read What Wikipedia is (and is not), so that you have an understanding of how to use, rely upon, or contribute to Wikipedia as you continue. Further information on key topics can be found below. If you can't find what you are looking for, try the Frequently Asked Questions or see Where to ask questions. For help with editing and other issues, see Help:Contents."