What is the difference between an unconditioned response and a conditioned response in Pavlov’s experiment?

In Ivan Pavlov’s experiments, the distinction between an unconditioned response and a conditioned response lies in what triggers the behavior.1

• Unconditioned Response: This is an automatic, natural reaction to an unconditioned stimulus. For example, when a dog is given food, it produces salivary secretion instinctively. Because the food invariably elicits this reaction without any prior training, the salivation is termed an unconditioned response.1

• Conditioned Response: This behavior occurs in reaction to a conditional stimulus—a previously neutral signal, such as the ticking of a metronome, that has been repeatedly paired with the unconditioned stimulus (food). The response is considered "conditioned" because its occurrence is dependent on the animal’s specific experience of associating the neutral signal with the arrival of food.1

In short, the unconditioned response is an innate reflex, while the conditioned response is a learned behavior triggered by a signal that predicts the arrival of the unconditioned stimulus.21