B.F. Skinner (1904 - 1990) is best known for developing the theory of Operant Conditioning, which uses reinforcers or consequences to change behavior.
According to this theory, the rate at which a certain behavior occurs is determined not by what precedes it, but by the consequence that follows it. For example, when a child puts away his toys, he gets praised by his parent. This positive consequence of the child's behavior will increase the likelihood that he will put away his toys after playing with them.
The key element to Skinner's theory is the reinforcer, which may be positive or negative. A positive reinforcer is one whose presence increases the likelihood of the response. A reward like food, money, or verbal praise are considered positive reinforcers. A negative reinforcer is one whose absence increases the likelihood of the response.