Unconditioned Stimulus:This is something that naturally and automatically triggers a response without any learning needed. For example, food is an unconditioned stimulus for a hungry animal.
Conditioned Stimulus:This is previously neutral but becomes meaningful after being paired with an unconditioned stimulus. In our analogy, the whistle becomes a conditioned stimulus after pairing it with the command to sit.
Neutral Stimulus:A neutral stimulus does not trigger any particular response until it's associated with an unconditioned stimulus. The whistle was initially a neutral stimulus for your dog.