The line between autobiography and memoir is fuzzy, and it is often difficult to distinguish between the two genres. In fact, some booksellers don't make the distinction at all and simply group autobiographies and memoirs together as nonfiction books written by the subject about the subject.
That said, however, there are some subtle differences between autobiographies and memoirs. Autobiographies tend to follow a chronological order and relate most of the events and interactions of a person's whole life. They are also usually focused more on facts rather than emotions and show how the subject fits into the larger scheme of history. An autobiography, even though it is written by the subject, often takes a more external perspective, and its language is usually (although not always) at least somewhat formal and objective.
A memoir, on the other hand, typically concentrates on a particular theme or era of the author's life. One writer might zoom in on a relationship; another might focus on his or her childhood or career. Rather than sticking to strict chronological order, memoirs tend to be more impressionistic, jumping around from time to time to illustrate changes in the writer's theme. A memoir's language is often less formal and more subjective, and anecdotes and emotions are frequently inserted. The writer's own memories and experiences are central with very little (if any) attention paid to the wider historical context.