Memoirs fall into three categories: 1) family legacy; 2) memoir; and 3) full-scale biography. Biography, however, does not technically encompass memoir, although the two terms are often used interchangeably. For more on the distinction, see my Biography page.
Memoir itself, however, falls into three different categories, but let's examine one term at a time.
Many people request a family legacy, which is intended to be an heirloom passed down through the generations of a family. It's not unlike a family tree with copious amounts of history added.
Family legacy is a fairly comprehensive book that encompasses more than just a single individual, although many such legacy books center on one family member, such as a patriarch or matriarch or earliest known ancestor.
These kinds of books chronicle entire families and include interviews with multiple family members. One of the most unique features of a family legacy is that the family portrait that emerges is set against a specific historical background. Good family legacy is always good history.
The books are usually sweeping sagas that provide wide-angle views of multiple generations that feed into a common ancestry. The scope, of course, can be reined in depending on the author's intentions since we can all trace our lineage back many generations thanks to online genealogy websites.
Memoirs focus on limited time periods and events in a person's life as opposed to a full account of an individual's experiences, from birth to the present. Notice that presidential autobiographies, however, are often called "presidential memoirs," and there again we see that the lines are blurred when using the two terms.
There are three categories of memoir and they all relate to the marketability of the book.
You should adjust your expectations according to where your projected memoir falls along the spectrum described above. That doesn't mean, however, that you shouldn't write your memoir. On the contrary, I believe, as stated in Your Story that everyone has a story to tell, that everyone is important.
The best reason to write a book is because you feel that it needs to be written--needs to "get out there." Some of the most famous books of all time were written without any publishing goal at all or with little hope of mainstream success. They were written because the author needed to express what had been pent up and internalized for too long.
That's why I say at the top of this site to dream big, write a book, and publish.