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
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.