Memoir is a writing genre that is often misunderstood. It is neither a biography nor an autobiography – see my Interview of a Memoirist published in WordWorks (pages 21-25), the magazine of the Federation of British Columbia Writers.
A memoir is not a biography, which is about the life of someone other than the writer. An autobiography is the work of someone sharing his/her life chronologically.
From the French language standpoint, it’s not as clear.
The plural form—mémoires—is the written account of the main events of one’s life.
The singular form—mémoire—is about narrating one main event from one’s life. It is also called récit personnel, which means personal narrative.
However, mémoire can also be a student’s final work demonstrating a learned expertise. At the doctorate level, it is called thèse—thesis.
Writers who specialize in the memoir genre are called memoirists.