Traditionally a documentary film is a film without a script or actors that portrays real people and events. Ideally the filmmaker tries to minimize their presence and instructions to the participants. It is often said that a documentary is made three times; when you conceive of the project, when you shoot the project and when you edit the project. At each phase there is an opportunity to discover the inner truth or veracity of the subject.
Some documentaries are short (under an hour) and others are feature length. Documentaries are rarely commercial, that is popular at the box office, but are usually made because the filmmakers feel strongly about a subject or topic, as is described as a "labor of love". Some however, like "Woodstock" (and others in the concert music vein) have done well and survived as "documents" of particular times and social melieus.
The true spirit of documentaries, as opposed to fictional films, is "the pursuit of the truth" which can be very difficult because all filmmakers bring their own unique point-of-view to the party and must excercise great discipline to remain objective. This is why many people dismiss Michael Moore's films (such as the hughly successful Farenheit) as not being "true" documentaries. Other filmmakers are sometimes more successful in hiding their agendas, even though their oppinions are just as strongly held.
With the proliferation of "reality television" the distinctions between fiction and non-fiction are now often confused, and even a sophisticated audience can be fooled by the use of documentary technique (like handheld camera and jump cutting) into thinking that a scripted show is real and true (Soderbergs HBO show about Washungton lobbyists comes to mind). It is therefore up to the modern audiences to be more decerning than ever about such things.
I hope this gives you some basis on which to pursue this topic!