With negative reinforcement the occurrence of a behavior is increased by removing an unpleasant stimulus. For example, your dog can avoid being spanked when it sits in response to your command. If the dog has been getting spanked, not getting spanked is rewarding (removal of unpleasant stimulus) so the frequency of the behavior will increase. People confuse negative reinforcement with punishment–just remember that with reinforcement you increase the occurrence of the behavior but punishment extinguishes a behavior.