What is the difference between classical and operant conditioning?

Classical conditioning is a reflex response which involves learning an association between a naturally relevant stimulus such as food (called the uncondtioned stimulus) and a stimulus which is not naturally relevant (the conditioned stimulus). Ivan Pavlov demonstrated classical conditioning in an experiment on his dogs: the dogs learned to associatiate the sound of a bell (unconditioned stimulus) with the presentation of food (conditioned stimulus). The dog's natural reaction on seeing the food was to salivate (unconditioned response) but after the period of conditioning, the dogs salivated simply upon hearing the bell (conditioned response). Operant conditioning involves a response which is instrumental in achieving the outcome. For example, if Pavlov’s dogs learned to press a lever in order to get their food, this would be operant conditioning. Furthermore, operant conditioning is modified by reward (positive reinforcement) or punishment (negative reinforcement).