Reinforcement is the process of following an event with a second event meant to make the recurrence of the first event more likely. The second event itself is called the reinforcer.
How do positive and negative reinforcement work?
Positive reinforcement is the presentation of a rewarding or pleasant stimulus that increases the probability that a particular response will occur. For example, if a student rewrites a term paper and is rewarded for that rewrite by a better grade, getting the grade is the positive reinforcer, and the teacher's awarding the grade to encourage rewrites is a positive reinforcement.
Negative reinforcement, on the other hand, is the presentation of an unpleasant stimulus that increases the likelihood that a particular response, in order to remove or avoid the negative reinforcer, will occur. For example, giving a rat an unpleasant electric shock when it presses a bar increases the probability that the rat will avoid the bar-pressing action.
Punishment differs from negative reinforcement in that it decreases the probability that a particular preceding event will occur again. When subjects are punished, they experience the unpleasant stimulus rather than avoid it.