A documentary film approaches a subject from a non-fictional angle, and many documentaries try to present their subjects in an unobtrusive, unbiased way that doesn't make judgments or attempt to lead the audience to predetermined conclusions. Other documentaries, however, have a very explicit agenda and seek to shape the audience's reactions to the subject.
In its purest form, documentary movies present facts without any attempt to influence how the viewer interprets those facts. In practice, documentaries can never achieve that purity, since the very act of choosing what to include in the film and how to present it introduces some of the filmmaker's bias into the final product.
Among those documentaries with an agenda, social and political documentaries are usually the most explicit in their points of view. Political documentaries in particular often take a particular viewpoint and construct a narrative to support that viewpoint.
Historical documentaries tell a story about specific time periods, historical events or historical figures. Military campaigns, exploration and cultural events are common subjects of historical documentaries. Although political events may be the subject of a documentary like this, a film with the intent of illustrating the historical context of the event is more likely to describe how and why the event took place and less likely to try to persuade the viewer toward a particular ideological conclusion about the event.
Another common type of documentary is the biography. These documentaries tell the story of a well-known individual. Governmental leaders, religious figures, sports stars, artists and musicians are typical subjects of biographical documentaries.
Finally, science-related documentaries attempt to explain scientific or technological topics. Like historical documentaries, science documentaries are often unbiased, although contemporary documentaries about the food industry or environmental issues often have a strong ideological lean in one direction or another.