The Oxford English Dictionary defines autobiography as
"An account of a person's life given by himself or herself, especially one published in book form. Also: the process of writing such an account; these considered as a literary genre".
An autobiographical essay is a short account of some aspect of the writer's life. It may include a brief description of the writer's experiences,hobbies,interests and some memorable events. Another term for an autobiographical essay is narrative essay. Since the autobiographical form is a first hand account of a person's life presented in its original form without interpretation or commentary from other writers, it is considered a primary source.
For additional information on this literary genre refer to Purdue's Online Writing Lab:
According to the Oxford English Dictionary, a biography is defined as
"The process of recording the events and circumstances of another person's life, esp. for publication (latterly in any of various written, recorded, or visual media); the documenting of individual life histories (and, later, other forms of thematic historical narrative), considered as a genre of writing or social history".
Since a biography is an account of a person's life written by someone else, it is considered to be a secondary source.
The OED defines memoir as "autobiographical observations; reminiscences".
Autobiographies and memoirs are similar in that they both are written in the first person and both are personal and talk about the author's life. The difference is while autobiographies detail in chronological sequence the author's life from birth to death, memoirs are concerened with emotional truths and focus on random aspects of the author's life such as feelings or attitudes that stand out because they have had such an impact on the person's life. The line between autobiographies and memoirs is fuzzy that they are often used interchangeably.