These books are for readers, just starting out. These are picture books that are fun to read, simple, short, and easy. They are meant specifically for very young readers, ages 4 - 6.
These books are for kids who are just getting introduced to chapter books, and these books aren't advanced, just a great book for beginner readers. A great example of this is Charlotte's Web or Land of Stories.
These books are for kids who can now read chapter books, and can be introduced to more advanced books and series such as the Chronicles of Narnia, or Artemis Fowl.
These books are for children who can now read competently, and are able to read more advanced books and series, like Lord of the Rings or Gordon Korman books.