A biography is an account of somebody's life written by somebody else, complete with details of the most important parts.
These days, anyone, of any age, can be the subject of a biography: Justin Bieber, at the tender age of 17, had one written about his life. A biography is not to be confused with an autobiography, an account of someone's life written by the subject himself. You'll find biographies in printed form (remember books?), but also increasingly in the form of e-books, TV dramatizations, and cinematic "bio-docs."