Edwin Thorndike. Thorndike discovered that cats learn faster if they are rewarded for their behavior (yeah- real genius concept). He called this idea the law of effect that states if the consequences of a behavior are pleasant, the behavior will likely increase. He also stated that is the consequences of a behavior are unpleasant, the behavior will not likely increase- and he called this whole idea, instrumental learning. Now his ideas are important because they were the springboard for the big mac daddy warbucks of operant conditioning, B.F. Skinner.
B.F. Skinner actually coined the term operant conditioning and is started this whole school by inventing the first operant conditioning chamber, otherwise known as the Skinner Box.
Positive Reinforcement: the addition of something pleasant to increase a behavior. If I want to to study more and give you chocolate for studying, the chocolate is the positive reinforcement because it is pleasant and meant to increase your behavior. Negative Reinforcement: the removal of something unpleasant to increase a behavior. If you have a headache and I want you to study, I may give you a Advil. The Advil is the negative reinforcement because it is removal something unpleasant (headache) and increasing your behavior (studying).
Positive Punishment: the addition of something unpleasant to make a behavior less likely. I want you to stop talking in class, so I flick you with a rubber band every time...