An autobiography is a written life story
With an autobiography, you try to summarize your own life story in a book. It is often written only when someone is in age, and when there is a retrospective of life. It takes quite a bit of research into your self, trying to figure out what are the most important experiences of your life now etc. It is certainly not a diary that is summarized, because a diary is written just after the events. When the story consists of some loose notes, or only a small part of life describes then there is often talk of a person's memoirs. The problem with an autobiography is often that a person who has an interesting life has not led a good story of it can write (or the other way: someone who can write well may be able to refine the actual events).
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