What is the difference...?

I think the best way to answer this question might be to find some books that are marketed as memoirs and some that are marketed as autobiographies and read them.

I just checked Webster, who says memoir is "a narrative of experiences that the writer has lived through." When plural - memoirs - an autobiography. So, an autobiography is a biography written by the subject rather than someone else, and would be expected to be a life story. A memoir is limited to a particular experience, event or period in the writer's life. That is how I interpret it. i have a couple memoirs about to go out to rejection land, one about childhood in a small town during world war two, the other a collection of experiences riding on steam trains bundled into one narrative poem.