A conditioned stimulus is previously neutral stimulus that, after becoming associated with the unconditioned stimulus, eventually comes to trigger a conditioned response.
This is the learned reaction to the conditioned stimulus. In our analogy above, feeling hungry when hearing the special ringtone would be your conditioned response.
This is when an organism responds in a similar way to different but related stimuli. For example, if you started feeling hungry when hearing any phone ring after conditioning.
This is when an organism learns to respond differently to various stimuli that are similar. For example, only responding (feeling hungry) to your specific pizza-ringtone and not other ringtones.