A biography is simply the story of a life. Very short biographies tell the basic facts of someone's life and importance. Longer biographies include that basic information of course, with a lot more detail, but they also tell a good story.
Biographies analyze and interpret the events in a person's life.
They try to find connections, explain the meaning of unexpected actions or mysteries, and make arguments about the significance of the person's accomplishments or life activities. Biographies are usually about famous, or infamous people, but a biograpy of an ordinary person can tell us a lot about a particular time and place. They are often about historical figures, but they can also be about people still living. Many biographies are written in chronological order. Still others focus on specific topics or accomplishments.