The Foundation's by-laws, which include the full mission statement, are available online at http://wikimediafoundation.org/wiki/Bylaws. However, a good summary of the mission is:
Developing and distributing online free, open-content collections of documents, information, and information databases in multiple languages.
The Wikimedia Foundation board of trustees are Jimmy Wales, Florence Nibart-Devouard, Angela Beesley, Tim Shell, and Michael Davis. There are brief biographies of each of the board members available.
Are there any officers?
The board has appointed officers at different times to fulfill needs, and the exact number of officers has changed over time as different needs are met or determined. Currently there are three officers: Technology, Research, and Legal. Brion Vibber is the technology officer, James Forrester is research officer, and Jean-Baptiste Soufron is legal officer.
The vast majority of Wikimedia's funding is through personal donations. The average donation is approximately 22 US dollars, which means there are many thousands of donations needed to keep the computers running.