what is the comparison between positive and negative reinforcement?

Answer 1

comparison = similarities

contrast = differences

http://www.answers.com/topic/positive-reinforcement
and
http://www.answers.com/topic/negative-reinforcement

Let us know what you think.

Answer 2

Positive reinforcement is giving a reward for some behavior.

Negative reinforcement is taking away some aversive stimulus for some behavior (e.g., quitting nagging if the person does a task).