For those who are hearing of wikipedia for the first time, here's an extract from the press release:
Started in January 2001, Wikipedia is the world's most current, fastest-growing, and largest encyclopedia. It is created entirely by volunteers who contribute, update, and revise articles in a collaborative process. The English language edition contains 20 million internal links, and incorporates 65,000 edits and 1,700 new articles each day.
Wikipedia's content is written for a general audience, and is continually revised for clarity, readability, and accuracy. Original text, images and sounds contributed to Wikipedia are licensed under the GNU Free Documentation Licence (GFDL), which lets users copy and modify each other's work based on a principle known as "copyleft." The entire database can be freely downloaded in full.
Though the project faced criticism in 2005 for factual inaccuracies in some articles, the science journal Nature published a study in December which found Wikipedia's science content to have only slightly more errors per article than that of the Encyclopedia Britannica. Other, less formal external peer reviews have been generally positive.
Congrats to all the authors. By the way, the millionth article was about Jordanhill Railway Station in Scotland.