A diary is a book which a person writes about what they have seen or heard or what they have been doing. Diaries are usually handwritten. People like to keep diaries (meaning: write a diary) for a variety of reasons: they may want to keep a record for themselves about what they have done during their life. They may sometimes want to publish it so that other people can read it. Some diaries may be important for business or military purpose. Children in schools are often asked to write a diary. This helps them to write about what they have been doing and expressing their thoughts.