In a sense, autobiography is a form of biography, the writing of a life story. The difference, of course, is point of view: an autobiography is from the viewpoint of its subject. Biographers generally rely on a wide variety of documents and viewpoints; an autobiography may be based entirely on the writer's memory.
A memoir is slightly different from an autobiography. Where an autobiography focuses on the "life and times" of the character, a memoir has a narrower, more intimate focus on his or her own memories, feelings and emotions.
For example, the autobiography of a Civil War general might include sections on the nature of slavery, the origins of the Civil War, and the political career of Abraham Lincoln. But the memoir of a Civil War general would focus on his personal reasons for joining the battle, the effect of the war on his mind and soul, and the joy and fear he felt on the battlefield.
Until the last 20 years or so, few people without some degree of fame tried to write and publish a memoir. But with the critical and commercial success of such memoirs as "Angela's Ashes" and "The Color of Water," more and more people have been encouraged to try their hand at this genre.