In American tradition and many Europeans cultures, children are told that Santa Claus separates people into two lists: the naughty list and the nice list. Those on the nice list receive lots of good gifts for Christmas; those on the naughty list get a lump of coal.
Supposedly, children who are well-behaved and good-natured throughout the year will be put on Santa's nice list. When children are naughty and misbehaving around Christmas time, some parents use Santa's naughty list as a warning that the children might not receive presents if they don't change their behavior.
Find out if you're on Santa's naughty or nice list at Claus.com.