Biographies are true stories, written about a real person. They can be about the person's entire life, or just one part of the person's life.
Biographies written by the topic person (usually written in 1st person) are called autobiographies. (If you wrote a story about your own life, it would be an autobiography.)
Biographies are often written using research on the person. The author can interview people, look up other books, use photographs, or get information from other places.