Memoir is a writing genre that is often misunderstood. It is not an autobiography, as per my Interview of a Memoirist published in WordWorks (pages 21-25), the magazine of the Federation of British Columbia Writers. You can also read that article in Other Stories.
A memoir is not a biography, which is about the life of someone other than the writer. An autobiography is the work of a writer sharing his/her life.
From the French language standpoint, it’s not as clear as it is usually in the plural form—mémoires—and is described as a type of biography based on history. It makes sense since mémoire also means memory. Writing a memoir is narrating authentic and personal events from one’s life memories.
In the singular form, the French word mémoire is the final work of a student demonstrating learned expertise. At the doctorate level, it is called thèse—thesis—and is the size of a book.
Writers who specialize in the memoir genre are called memoirists.