Documentary films, commonly referred to as 'documentaries' or 'docs,' differ considerably from narrative or fiction feature films, even those with stories that are based on real life events.
The primary difference is that documentary films are obliged to stick to the truth--without alteration or elaboration.
Truth Be Told
To show and tell the truth, documentary filmmakers use special resources and techniques. They present montages of current and archival film footage that explores specific subjects, including events past and present, natural phenomena, profiles of famous people, culture and the arts, and all other imaginable topics. If there is anything in the real world you're interested in, there is probably a documentary film to show and tell you more about it.
Style and Point of View
That said, documentary films vary significantly in style and point of view. Conventional docs, such as In Search of Mozart, use moving and still images to introduce you to their subject--the life and music of young Wolfgang Amadeus Mozart, in this case.
At the other end of the style spectrum, you'll find the work of experimental documentary filmmaker Abigail Child, whose Mayhem is a highly personal expression that uses unexpected juxtapositions of images.
Some documentary films feature their own filmmakers--Michael Moore in Sicko, for example.
Others use reenactments to tell what has happened when actual footage isn't available. Michael Winterbottom's Road To Guantanamo is an example of that technique.
Powerful Propaganda
Documentaries can have powerful social and political influence. While watching them, try to be sensitive to any possible bias in the director's point of view and about the film's inherent propaganda value. In her classic documentary, Triumph of the Will, director Leni Reifenstahl praised Adolph Hitler. Great film, questionable message.
On the other hand, Amy Berg's Oscar-nominated Deliver Us From Evil, a powerful exposé of child abuse by the Catholic clergy in Los Angeles, may have influenced a $660 million settlement offer the Los Angeles Archdiocese has made to abuse victims.
Conflict of Interest
When watching documentary films with possibly biased points of view, be mindful about funding sources. Did a tobacco company pay for that lovely film showing cigarettes as lifestyle essentials in the context of Twentieth Century figurative paintings? Perhaps you might want to consider the fact that The Dixie Chicks hired Barbara Kopple to direct Shut Up & Sing, the film in which they return to center stage from the outskirts of ostracism, following their stand on the Iraq War. That information might make the film's message seem a bit less credible.
Ethics and Accountability
Conflict of interest and questionable credibility aren't the only standards by which documentaries are judged. Another element to consider is the filmmaker's use of living subjects, and responsibility for portraying them accurately. In Tootie's Last Suit, filmmaker Lisa Katzman follows Mardi Gras 'Indians' in battle to determine whose costume is 'prettiest.' The film reveals rivalry between Tootie and his son that both subjects might find painful to watch, but Katzman cannot be faulted for betraying their trust.
Evolution of Documentary Filmmaking
Documentary filmmaking has evolved over the decades. Early examples, such as Nanook of the North, required cumbersome equipment that rendered fascinating but imperfect images. Today's digital technology allows filmmakers--professionals and hobbyists--to go for instantaneous guerilla-like image capture that can reveal truths about subjects who may be unaware they're being filmed.
More popular than ever
Documentary films are becoming increasingly popular with everyone. Fledgling filmmakers find them a more affordable way to enter the profession. Audiences find them a more interesting and credible source of information than what's produced by most mainstream media outlets that have cut their long form reportage and concentrate more and more on news fluff rather than on news substance.