The CIA, created in 1947 by President Truman, is a descendant of the OSS of World War II. <more information needed here>
The activities of the CIA are, obviously, largely not disclosed, but some educated guesses can be made about its activities. Like other intelligence agencies, it collects information from a variety of sources, the vast majority probably being public information in the countries concerned, but also from individuals who for various reasons including bribes, blackmail, and ideology, decide to pass otherwise secret information to the CIA. It also undoubtedly makes use of the surveillance satellites and signal interception capabilities of the NSA, including the Echelon system, and the surveillance aircraft of the various branches of the US armed forces. At one stage, the CIA even operated its own fleet of U-2 surveillance aircraft.
The CIA's work has been fictionalised with varying degrees of accuracy in a considerable number of spy novels.
One of the CIA's publications, the CIA World Factbook is unclassified and is indeed made freely available without copyright restrictions. The factbook forms the basis of most of the country entries in this Wikipedia.