A documentary for me is to document the reality about a subject through the medium of film. Documentaries usually tell their audiences the facts, often illustrating the main points using visual aids i.e. powerful and thought provoking footage.
A powerful documentary should inform people and open their minds to the different opinions surrounding that subject, inspiring people to form an opinion of their own.
Louis Theroux is a journalist who works in the medium of film and who inspires me because of his unique way of informing his audiences. For me his subject matters are utterly intriguing and above all, eye-opening: http://www.youtube.com/watch?v=u9mi1RX7Ttg
