Simply put, a memoir is a genre of writing that gives an account of your life.
Within the genre, there are different types of memoir. You could write a travel memoir, or a spiritual quest memoir, or, the most well-known of all, a personal memoir. The subset within memoir is further defined by your memoir's focus and specific perspective.
For example, in his personal memoir A Moveable Feast, Ernest Hemingway writes about a period of five years (1920–1925) when he first lived in Paris with his first wife, Hadley. In the memoir, he makes subtle references - here and there - of what was to follow in his life. But his focus is on his experience as a struggling novice writer who is new to the expatriate writing community in Paris in the 1920s. His wife and child are referenced, and some dialogue with Hadley is included, but the central perspective of this memoir is his interaction with his emerging literary circle.
He writes about his friendships with Gertrude Stein, Ezra Pound, and F. Scott Fitzgerald. He invites us into the cafés of Paris where he writes, drinks wine and lunches with other literary personalities. At times, we learn more about James Joyce, Ford Madox Ford, and even the server at the Lilas, than we do of his family.
This illustrates two aspects of memoir writing. First, choose your focus and stay true to it. Second, you can write more than one memoir.
Hemingway could have chosen to write a second memoir with the focus being his relationship with his wife over the same five years. Or he could have written a memoir of his time as a war correspondent in the Spanish Civil War, or when he lived in the Florida Keys.
Another approach to writing a personal memoir may be to explore significant relationships in your life. In her memoir The Glass Castle, Jeanette Walls tells the story of her relationship with her unconventional parents, her creatively absent mother and her alcoholic father. It is a heartfelt exploration of the deep complexities in familial relationships. Her memoir spans her childhood into her adulthood.
All memoirs are an account of someone's personal life, knowledge, and experience. What is yours? Which type of memoir have you written? Or which is the one you are thinking of writing?