People who tell you not to eat candy while they chomp away on licorice all day? People who say they hate cars but always beg you for a ride? They are engaging in hypocrisy, or behavior that is different from what they say they believe.
A hypocrite is a person who practices hypocrisy: what they say is not what they do. The noun hypocrisy descends from the Greek hypokrisis "acting on a stage," from hypokrinesthai "to play a part, pretend," from the prefix hypo- "under" plus krinein "to judge." Many times kids are enraged by their parents' hypocrisy when parents make their children follow rules they don't follow themselves.