In some general contexts, memoir and autobiography can be used interchangeably. However, when publishing houses note a difference, they're making the following distinction: autobiography focuses on the writer's entire life, whereas memoir focuses on a certain aspect of it.
As Susan Carol Hauser explains in You Can Write a Memoir, autobiography focuses on the chronology and timeline of a life. Memoir is concerned with themes that occur within that chronology.
For example, the guiding structure for The Autobiography of Benjamin Franklin is the chronology of Franklin's life. It is a near complete history of his existence—it doesn't focus exclusively on one particular aspect (his romances or his parents or the development of his life philosophies), but is rather an all-encompassing view of the man.
On the other hand, the memoir Prozac Diary is Lauren Slater's meditation on the effects of Prozac in her life—how it helps her deal with her mental illness and how it both confuses and clarifies her own identity. It covers other aspects in her life, her relationships, her jobs, etc., but only as they're related to her experience with mental illness and Prozac. It is an in-depth look at one theme in her life, rather than an in-depth look at her entire life.