An "Elf on the Shelf" is a small stuffed doll dressed as Santa's elf that comes with the book "The Elf on the Shelf: A Christmas Tradition." The elf comes to visit the child's home every day during the Christmas season.
The elf is there to report to Santa Claus on the child's behavior, according to the book. He flies back to the North Pole each night, returning to the home every morning before the child awakens. He appears in a different location in the house each day, and the child has to find him, but must never touch him.