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.
Some people, such as Samuel Pepys, have become famous as a diarist. Pepys lived at an interesting time (he was alive at the time of the Great Fire of London). We know a lot about what London was like by reading his diary. Anne Frank wrote a diary while she was hiding from the Nazis.
The word diary can also be used for a pocket diary: a small book in which people can write down their appointments so that they can remember what they are supposed to be doing each day.