An autobiography generally covers the writer's entire life up to the point of writing the autobiography, while a memoir focuses on specific aspects or periods of the writer's life. Though these two genres are typically shelved together by bookstores, they are in fact quite different.
When a writer organizes an autobiography, he focuses on hard facts like when and where he was born, what his life was like or what his accomplishments were. In a memoir, however, a writer generally writes to a theme, such as what it was like to overcome addiction or live through the Russian revolution. Hard facts are not so critical, but the storytelling aspect rises in importance. In addition, an autobiography is more likely to be written by a famous person, while memoirs can be written by anyone with a fascinating story.