Neutral Stimulus: A neutral stimulus is something that does not naturally elicit a response before conditioning. For example, the sound of a bell ringing before it is associated with food.
Conditioned Response: A conditioned response is a learned response that occurs as a result of an association between a previously neutral stimulus and an unconditioned stimulus. In this case, salivating at the sound of the bell ringing after it has been paired with food.
Unconditioned Stimulus: An unconditioned stimulus is something that naturally elicits an automatic response without any prior learning. In this case, the presentation of food which naturally causes salivation.