what is the story with the elf on a shelf

The Elf on the Shelf is a popular children's book that tells the story of a small elf who lives in a house with a family. The elf is sent by Santa to keep an eye on the children to ensure they are behaving and to report back to him on their behavior. The elf is said to have magical powers that allow him to move around the house without being seen, so the children have to be careful not to misbehave or else the elf will report back to Santa.