There are two types of reinforcement: (1) Positive reinforcement (2) Negative reinforcement.
(1) Positive reinforcement: Positive reinforcement is a stimulus which increases the probability of response, when presented after the response. It increases the rate of response that precedes its presentation. For Skinner, hungary rat gets food pellet after it presses the bar, as a result the probability of bar pressing response increases. Positive reinforcers have pleasant sequence as they strengthen and maintain the response upon which they are contingent, e.g. A mother giving a chocolate to her child on finishing his homework. Chocolate is a positive reinforcement for the child.
(2) Negative reinforcement: Generally, we consider positive reinforcement as a reward while negative reinforcement as punishment. But Skinner does not consider negative reinforcement as punishment. It is a stimulus which increases the possibility of response when removed after the response. Thus negative reinforcer is used to increase the probability of escape or avoidance response. For example a person wears woolen clothes to protect himself from cold.