Difference between classical and operant conditioning
In classical conditioning, a conditioned stimulus (such as a bell ringing) is paired with a stimulus that is already hard-wired to evoke a response (such as food evoking salivation) until the conditioned stimulus evokes the same response.
In operant conditioning, behavior is reinforced by reward and the best way to do this is with positive reinforcement given at an intermittent (variable) schedule (like how casinos reward gambling).