Biographies are accounts of someone's life written by another person. They are considered secondary sources because they were written by someone other than the subject.
Autobiographies are accounts of someone's life written by that person. They are considered primary sources.
Memoirs are historical accounts or biographies written from personal knowledge or special sources. They are different from autobiographies due to their often emotional tone focusing on events in the author's life that were significant to them emotionally. Autobiographies are often more chronological and factual in tone.