Classical Conditioning was used by Ivan Pavlov to show how a signal can stimulate a response such as a bell. Here the dog associates the sound of a bell to food.
Operant Conditioning, on the other hand, was used by B.F Skinner. It associated reinforcement and punishment with a certain desired behavior. For example, a dog eagerly gets the ball because it gets something in return. Punishment can be a ground for not having something that a person wants, so the person avoids breaking the rules.