The conventional explanation of the difference is that a memoir focuses on one (or a few) limited aspect(s) of a person’s life: my time as a POW, the year I spent at sea, my vacation on Mars, life with Donald Duck, etc. On the other hand, an autobiography is more generally thought of as a chronological recounting of an entire life.
But when you boil it all down, what it’s really about is removing the mask(s) and revealing at least a part of yourself to the world. Diary of a Ninja might make a good title for a novel, but I suspect the real thing wouldn’t happen. Why? Well, because… ninja. C’mon, geez.
For some people, the essential part of their life is the end product of their entire life. Okay then, how does that jibe with the explanation above? Short answer: it doesn’t. The more important answer, I believe (with apologies to any library science majors who might find this frustrating), is that it doesn’t matter what you call it. I’d strongly suggest, however, that you make the distinction in or near your title. Or at the very least, on the back cover.
So, if you’re writing about your life as an art critic, your title might run along the lines of No Paint, No Pain — How I Skewered Art, Without Letting Art Skewer Me. An instant bestseller? Probably not. But at least readers will have an idea of what you’ve written about. On the other hand, if your book features the struggles you faced and the battles you won despite having a second head, your title might read something like Conversations With Myself — Two Heads Aren’t Always Better Than One.
Some readers may think this whole topic is just silly, not worth the energy required to think about it. And yet I know quite a few folks who worry not just about this, but whether their story — their memoir — should be titled in the singular or the plural: memoir or memoirs? According to Merriam-Webster (my interpretation anyway) is that no one cares. If you write a memoir about your life as a ski instructor (you lucky so-and-so), and follow it with the years you worked as a lifeguard for Las Vegas showgirls (poor baby), then you’ve written two memoirs. If you combine it all — perhaps: Surf and Snow: My Life of Perpetual Pleasure — then it’s one memoir.
Anyway, that’s my take. More soon.