The author's purpose is the main reason or reasons why an author writes about a particular topic. Authors bring out their purpose through different sorts of writing formats, genres and languages. An author's purpose can be to persuade or convince, inform or entertain the readers.
If the author's goal is convincing the reader to agree with her, the purpose is to persuade. If an author writes texts to enlighten the reader about real topics, the purpose is to inform. Lastly, if the author writes a story to make the readers enjoy what they are reading then the author's purpose is to entertain.