To me, an autobiography (or biography) is more a list of things that happened and when they happened. A memoir (again, to me) is more selective. Instead of writing about everything, the author picks out certain memories, those that pertain to the story, leaving a vast majority out sometimes skipping over huge portions of a person's life. It also doesn't have to be linear but can jump back and forth in time as the story dictates. In structure it's more like a novel than an autobiography would be.