"Wikipedia is a free Encyclopedia written collaboratively by the people who use it. It is a special type of website designed to make collaboration easy, called a Wiki. Many people are constantly improving Wikipedia, making thousands of changes per hour. All of these changes are recorded in article histories and recent changes." (From Wikipedia)
For more information about these principles, please check out the original Wikipedia Page.
Women and minorities are underrepresented on Wikipedia, in the arts and in STEM-related fields. Here are links to information on how people have used or are using Edit-a-Thons to attempt to redress the balance.
Kenneth Sherwood's apt summary of the usefulness of Wikipedia in DH (you'll need to scroll a bit).