Q: What do you call an elf who sings?
A: A wrapper!

Explanation: In this joke, elves are Santa Claus’ helpers.  They are the ones who make the toys and wrap them so that Santa can deliver the toys on Christmas Eve.  “To wrap a present” means to cover it with decorative paper to make the gift look nice.  Wrapping a present also makes people wonder what is under the wrapping paper.

A “Rapper” is a person who sings, usually songs that have amazing rhymes at high speed.  (For more information on the history of rap music check out this link.)

This joke is funny because “wrap” sounds like “rap.”

Here is a Christmas rap song about the Grinch Who Stole Christmas: