The CIA World Factbook is an annual publication by the Central Intelligence Agency of the United States with basic almanac-style information about the various countries of the world. The factbook gives a two- to three-page summary of the demographics, location, telecommunications capacity, government, industry, military capability, etc, of almost all countries in existence.
The World Factbook is prepared by the Central Intelligence Agency for the use of US Government officials, and the style, format, coverage, and content are designed to meet their specific requirements. Information is provided by:
- Antarctic Information Program (National Science Foundation),
- Bureau of the Census (Department of Commerce),
- Bureau of Labor Statistics (Department of Labor),
- Central Intelligence Agency,
- Council of Managers of National Antarctic Programs,
- Defense Intelligence Agency (Department of Defense),
- US Department of State,
- US Fish and Wildlife Service (Department of the Interior),
- US Maritime Administration (Department of Transportation),
- National Imagery and Mapping Agency (Department of Defense),
- Naval Facilities Engineering Command (Department of Defense),
- Office of Insular Affairs (Department of the Interior),
- Office of Naval Intelligence (Department of Defense),
- United States Board on Geographic Names (Department of the Interior), and
- other public and private sources.
Federal law prohibits use of the words "Central Intelligence Agency," the initials "CIA," the seal of the Central Intelligence Agency, or any colorable imitation of such words, initials, or seal in connection with any merchandise, impersonation, solicitation, or commercial activity in a manner reasonably calculated to convey the impression that such use is approved, endorsed, or authorized by the Central Intelligence Agency. 
Many sites have used information in the CIA World Factbook, because of its public domain status, including Wikipedia.
Besides the World Factbook, the CIA puts out a directory of Chiefs of State and Cabinet Members of Foreign Governments each week.
See also: Abbreviations used in CIA World Factbook, Wikipedia:Status of the porting of the CIA World Factbook