Documentary photography usually refers to a popular form of photography used to chronicle events or environments both significant and relevant to history and historical events as well as everyday life.
It is about a particular subject or a story that a photographer wants to highlight. They are all objective pictures and can be about a war, social issues, science and so on. When you are chronicling significant events, for instance, the life of a celebrity or a sportsman, plan to spend some time with the person; chat; listen and capture what comes naturally. Try and capture details and let your pictures tell a story. Do not try to shoot something that is not a part of his or her life.