Difference between "memoir" and "biography"

A memoir is a sub-genre of the autobiography. As Wikipedia writes:

A memoir (from French: mémoire/ Latin: memoria, meaning memory, or reminiscence), is a literary genre, forming a subclass of autobiography – although the terms 'memoir' and 'autobiography' are almost interchangeable.

The criteria for determining whether a work is a memoir include:

• They tend to encompass only time period of the author's life
• They tend to deal with one part of the author's life--say, his career
• They are generally written in first-person

A biography, on the other hand, is a broader category. From Wikipedia:

A biography is a detailed description or account of someone's life. More than a list of basic facts (education, work, relationships, and death), biography also portrays the subject's experience of those events. Unlike a profile or curriculum vitae (résumé), a biography presents the subject's life story, highlighting various aspects of his or her life, including intimate details of experience, and may include an analysis of the subject's personality.

Biographies tend to be about someone's entire life, although a memoir is still a type of biography. The library would shelve them together because of their shared overarching category.

Simply put, a memoir is usually written by the person whose life the writing describes, while a biography may be written by another person.

Actually, the word "memoir" has two meanings: 1) a written account of somebody's life written by that person; 2) a written account of somebody's life written by a close person. And a biography not 'may be' but for sure is written by another person. If it is written by you, such a piece of writing is called an autobiography. For example, this military biography template could be either a biography or an autobiography and it all depends on the person who wrote it.