What Makes a Film a Documentary?

In general, documentary films focus on real life and include footage of events as they happened. A movie about World War II might feature actors portraying soldiers, real or fictional, in the war, recreating certain battles or events. In contrast to this, a documentary film about World War II might primarily feature news reel footage of actual fighting, with commentary from experts and veterans who were in the war. It is this focus on documenting reality above drama or a fictional narrative that typically separates these movies from summer blockbusters and other popular films.

I am facing some problems in my assignment What Makes a Film a Documentary. Can anybody suggest me the proper explanation for it?