Both these terms point to a similar genre: Books about one’s own life, written from one’s own perspective. Yet the word “autobiography” points toward a biographical summary of one’s life, written by oneself. It generally begins at birth and continues through the present day, recording one’s accomplishments and failures, trials and successes, over a long span of time. It captures a lot of life information and preserves it for future generations.
The term “memoir,” on the other hand, is more often used to describe books that cover a specific period of time, or a particular experience or circumstance in one’s life. For example, a woman in her 30s might write a memoir of the first year of motherhood. While she might draw in scenes from her own childhood, the focus of the book would be about the ups and downs of this specific year. In that way it could not be considered a biographical rendering of her entire life—it’s more a snapshot in time. It’s usually written to entertain and inspire the reader, rather than to provide biographical facts, dates and details.