Compilation Film
This type of film consists solely of file footage: moving images taken from other films, television shows, and commercials, and possibly still images from other sources. Using the amassed images, the filmmaker then edits them together, constructing a new idea or point outside their original intended meanings.
Almost as old as the newsreel itself, the compilation film is that subgenre of the nonfiction film that begins on the editing table with footage that was made for another purpose. In the hands of those who would edit or manipulate such newsreel footage for their own purposes, often distorting the work of another for their own monetary gain, the compilation film can exploit the commercial purpose of cinema. But in the hands of the original artists with a vision of their own, the compilation film has both a history and an aesthetic validity of its own.
Another recent development in the field of documentary is the creation of compilation films: for instance, "The Atomic Cafe" is made entirely out of found footage which various agencies of the U.S. government made about the safety of nuclear radiation (e.g., telling troops at one point that it's safe to be irradiated as long as they keep their eyes and mouths shut). "The Last Cigarette", a second example, combines the testimony of various tobacco company executives before the U.S. Congress with archival propaganda extolling the virtues of smoking.
SOURCES:
http://www.realityfilm.com/study/genreoverview.php
Richard Barsam. Nonfiction Film. A Critical History. p. 75
http://www.free-definition.com/Documentary-film.html