A documentary is considered as a media production, be it film, television or radio, that provides a factual report on a particular subject.
Bill Nichols is an American film critic and theoretician who is best known for his pioneering work as the founder of contemporary study on documentaries. Nichols said that:

Expository

An expository documentary has a disembodied and authoritative voice over that directly addresses the audience on what is happening in the accompanying images. The narration gives meaning to the images, not opinion. Captions are also used to reinforce facts and arguments provided by the voice over. The effect of such a documentary is one of objectivity as well as direct and transparent representation. An example of an expository documentary is the 2005 film, March of the Penguins.  

Observational

An observational documentary is unobtrusive, it presents a slice of life. The filmmaker doesn't intervene in what is being shown, they are hidden from the audience and are therefore an uninvolved bystander. Observational documentaries presents a transparent record of an event and is neutral and non-judgmental. The audience decides on what to think or decode from the images shown. An example of an observational documentary is Salesman, from 1968. 

In an interactive documentary, the filmaker's presence is evident, they are an active participant. There is interaction between the interviewees, the presenter and the audience. Agendas are usually presented and manipulation of the text through editing is much more evident. Interactive documentaries are criticized for misrepresentation and manipulation. An example of an interactive documentary would be Exit Through the Gift Shop.

Reflexive 

In a reflexive documentary the filmaker attempts to expose the conventions of documentary to the audience. The construction of documentary are exposed and experiemented with. The text will consist of shots that have captured 'everyday life' but are used as bricks which can make vastly different/experimental films. An example of a reflexive documentary would be Stories We Tell.

Preformative

This represents the world indirectly and emphasises presentation rather than content. Re-enactments, exaggerated camera position and soundtracks submerge the audience into the diegesis. Preformative films are often engaging, entertaining and enlightening.  Preformative documentaries aim to present the subject matter in an subjective, expressive, stylised, evocative and visceral manner. A filmmaker may want to control how facts are delivered and hopefully make the audience interpret the facts to meet the filmmaker's own agenda. This manner of dramatisations and re-enactments can exaggerate bias, the filmmaker's perspective becomes presented as fact. The personal point-of-view of interviewees may become private or disassociated from social perceptions, and preformative films often become reliant on the filmmaker's own voice. An example of a preformative documentary would be the 1989 film, Tongues Untied.

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