A Documentary is a non-fictional motion picture intended to document some aspect of reality, primarily for the purposes of instruction, education, or maintaining a historical record , simply put it is a television or radio programme, or a film, which shows real events or provides information about a particular subject.
Documentaries can be funny, poignant, disturbing, ironic, absurd, inspirational, amusing, shocking or any combination.It is a genre of movie making that uses video & film scenes, photographs and/or sound of real people and real events which when edited together creates a particular story, viewpoint, message or experience. Traditionally, documentaries are 30-minutes to 2 hours in length (to fit within a television schedule or for theatrical release). However, documentaries are often shorter in length, especially in recent years with the advent of the Internet and web video.
The documentary genre has radically evolved in recent years and so it's not a surprise that there may be some confusion about "what exactly is a documentary!". A documentary is a broad term to describe a non-fiction movie that in some way "documents" or captures reality. Documentaries are often used to reveal an unusual, interesting or unknown angle. Topics are limited only by one's imagination.
Documentary filmmakers are often motivated to make their films because they feel a particular story or viewpoint is not being (adequately) covered by mainstream media.