What is a documentary film?

Documentary films constitute a broad category of nonfictional motion pictures intended to document some aspect of reality, primarily for the purposes of instruction or maintaining a historical record.^[1] A 'documentary film' was originally shot on film stock — the only medium available — but now includes video and digital productions that can be either direct-to-video, made as a television program or released for screening in cinemas. "Documentary" has been described as a "filmmaking practice, a cinematic tradition, and mode of audience reception" that is continually evolving and is without clear boundaries.^[2]

http://en.wikipedia.org/wiki/Documentary_film