what is a documentary ??....

Documentaries bring viewers into new worlds and experiences through the presentation of factual information about real people, places, and events, generally -- but not always -- portrayed through the use of actual images and artifacts. But factuality alone does not define documentary films; it's what the filmmaker does with those factual elements, weaving them into an overall narrative that strives to be as compelling as it is truthful and is often greater than the sum of its parts.

Documentaries can be funny,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.
You also get documentaries has on TV channels and films.