Intelligence (abbreviated int. or intel.) is information valued for its currency and relevance rather than its detail or accuracy —in contrast with "data" which typically refers to precise or particular information, or "fact," which typically refers to verified information. Sometimes called "active data" or "active intelligence", these typically regard the current plans, decisions, and actions of people, as these may have urgency or may otherwise be considered "valuable" from the point of view of the intelligence-gathering organization. Active intelligence is treated as a constantly mutable component, or variable, within a larger equation of understanding the secret, covert, or otherwise private "intelligence" of an opponent, or competitor, to answer questions or obtain advance warning of events and movements deemed to be important or otherwise relevant.
As used by intelligence agencies and related services, "intelligence" refers integrally to both active data as well as the process and the result of gathering and analyzing such information, as these together form a cohesive network (cf. "hive mind"). In a sense, this usage of "intelligence" at the national level may be somewhat associated with the concept of social intelligence —albeit one which is tied to localized or nationalist tradition, politics, law, and the enforcement therof.

Well-known national intelligence organizations

Open Sources Center, select "Intelligence" section.
Bloomberg
CIA World Fact Book
credit rating agencies
Dow Jones
Internet search engines such as Google
LexisNexis
newspapers of record, such as the New York Times
private investigators
public libraries Major publicly accessible intelligence sources

Surveys

Beesly, Patrick. Room 40. (1982). Covers the breaking of German codes by RN intelligence, including the Turkish bribe, Zimmermann telegram, and failure at Jutland.
May, Ernest (ed.) Knowing One's Enemies: Intelligence Assessment before the Two World Wars (1984)
Tuchman, Barbara W. The Zimmermann Telegram (1966) World War I

Babington-Smith, Constance. Air Spy: The Story of Photo Intelligence in World War II (1957)
Beesly, Patrick. Very Special Intelligence: The Story of the Admiralty's Operational Intelligence Centre—1939–1945 (1977)
Hinsley, F. H. British Intelligence in the Second World War (1996) abridged version of multivolume official history.
Jones, R. V. The Wizard War: British Scientific Intelligence 1939–1945 (1978)
Kahn, David. Hitler's Spies: German Military Intelligence in World War II (1978)
Kahn, David. Seizing the Enigma: The Race to Break the German U-Boat Codes, 1939–1943 (1991)
Lewin, Ronald. The American Magic: Codes, Ciphers and the Defeat of Japan (1982)
May, Ernest (ed.) Knowing One's Enemies: Intelligence Assessment before the Two World Wars (1984)
Smith, Richard Harris. OSS: The Secret History of America's First Central Intelligence Agency (2005)
Stanley, Roy M. World War II Photo Intelligence www.softecare.com (1981)
Wark, Wesley. The Ultimate Enemy: British Intelligence and Nazi Germany, 1933–1939 (1985)
Wark, Wesley K."Cryptographic Innocence: The Origins of Signals Intelligence in Canada in the Second World War", Journal of Contemporary History 22 (1987) Cold War Era: 1945–1991

Compartmentalisation
Counter-intelligence
Espionage
Intellipedia, a classified wiki that runs the secret network that links the U.S. intelligence community.