Operant and classical conditioning are both types of learning. Though they are similar in the way they use stimuli to determine a specific learned outcome, classical and operant conditioning are different in the way they achieve the learned outcome.
Classical Conditioning, mistakenly discovered by Ivan Pavlov, is a learning process that occurs when two stimuli are repeatedly paired until a response is elicited from the unconditioned stimulus. Here is some vocab to help with understanding:
US: Unconditioned Stimulus: Something that naturally causes something else
UR: Unconditioned Response: Natural reaction to something (US)
CS: Conditioned Stimulus: Something that starts as neutral that eventually produces a learned response
CR: Conditioned Response: Behavior that comes from CS alone--a learned association
Pavlov's famous experiment is a great way to examine how classical conditioning works. Pavlov was doing an experiment with dogs and he used a bell (US) to let the dogs know that their food (steak) was coming. Over time, he realized that the dogs would begin to salivate at the sound of the bell because they had learned to pair it with the arrival of steak. So the bell (US) which previously produced no response, when paired with the steak (CS) which made the dogs salivate (CR) eventually came to make the dogs salivate! In this experiment the bell became the (CS) and salivation became the (CR). Knowing this, Pavlov was able to apply it to many other areas of learning.
Operant Conditioning is a bit different. First discovered by B.F. Skinner, operant conditioning is when a particular consequence produces the desired outcome. There are different types of consequences: reinforcement and punishment. Reinforcement is used when an experimenter wants to increase behavior, which can be done by adding the desired response (positive reinforcement) or taking away an undesired response (negative reinforcement). An experimenter would use punishment if they wanted to decrease behavior.
Some of the major differences between the two are that classical conditioning focuses on involuntary behaviors, whereas operant conditioning focuses on strengthening or weakening a behavior that already exists. Another difference is how they use stimuli; classical conditioning places the stimuli before a reflex (behavior), whereas operant conditioning applies the consequence after the behavior.