From Wikimedia Foundation:
Wikipedia 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 Web site and encyclopedia. Since its creation in 2001, Wikipedia has grown rapidly into one of the largest reference Web sites.