The terms memoir and autobiography both refer to what people write about themselves. So what's the difference between the two words? The answer to that question depends on whom you ask.
William Zinsser edited a book entitled Inventing the Truth: The Art and Craft of Memoir. The book resulted from a series of talks sponsored by the Book-of-the-Month Club, Inc., and held at The New York Public Library in the winter of 1986. In his introductory section Zinsser says that, for this project, memoir was defined as some portion of a life. He continues:
Unlike autobiography, which moves in a dutiful line from birth to fame, omitting nothing significant, memoir assumes the life and ignores most of it. The writer of a memoir takes us back to a corner of his or her life that was unusually vivid or intense--childhood, for instance--or that was framed by unique events. By narrowing the lens, the writer achieves a focus that isn't possible in autobiography; memoir is a window into a life. (p. 21)
In other words, an autobiography aims at breadth, while a memoir focuses on depth. A good example of this distinction is Joan Didion's memoir The Year of Magical Thinking, in which she describes the process of trying to adjust to the death of her husband and constant companion of 40 years.
This is a sometimes useful distinction, but it is not a universally accepted one. For our purposes here, memoir and autobiography are synonyms.
Jill Ker Conway, in When Memory Speaks, describes the appeal of autobiography in this way:
the answer to the question of why we like to read it [autobiography], and why individuals sit down at desk or table and begin to tell their story, lies not in theory but in cultural history. It has to do with where we look when we try to understand our own lives, how we read texts and what largely unexamined cultural assumptions we bring to interpreting them. (p. 4)
Reading autobiography is a way to expand our knowledge of human experience. Writing autobiography is a way to define ourselves, to contribute to the body of human knowledge, and to pass on something of ourselves to future generations.