The key way in which biographies and autobiographies differ is in their author, as a biography is written about a person by someone else, while an autobiography is written about a person by that person. Both biographies and autobiographies provide essential information about their subject. Autobiographies are also often called journals or memoirs.
Autobiographies and biographies are also different because they have different points of view. With biographies, the account must be written in third person, because it is written by someone who did not experience the events in the story and only narrates it. In autobiographies, the writer uses first person, because he or she is talking about personal experiences.
In addition, there are different levels of subjectivity in autobiographies than there are in biographies. In an autobiography, all events are recounted as they were experienced by the author. This allows the author to give readers a better sense of his persona and language, but it also means that the author may not be able to be trusted for a completely unbiased opinion. A biographer, on the other hand, is much more likely to be objective because he is basing the work on extensive research performed on the subject of the book and not personal opinion.
In an autobiography, the writer is able to directly address and speak to readers in a way that seems immediate and real. However, this only allows the reader to see the person's life from one view, which may be skewed. A biography offers many more viewpoints of the person that it covers.