All of these phenomena are related to the theory of classical conditioning that was presented by Ivan Pavlov who is a renowned name in the field of psychology for his contributions to classical conditioning. A stimulus, in the elementary comprehension of the word, is something that has the tendency to ignite a reaction.
A neutral stimulus is a stimulus that does not have any effect on a person before he is conditioned (classically) to display a reaction to it. As for an unconditioned stimulus, it is in contrast with the neutral stimulus and has a tendency to illicit a response from the subject even without the aegis of conditioning. In relation to an unconditioned stimulus, an unconditioned response has pretty much the same meaning except that it is a response, which is an innately emerging one rather than being catalyzed with the support of any sort of conditioning or learning.
A neutral stimulus is a stimulus that does not have any effect on a person before he is conditioned (classically) to display a reaction to it. As for an unconditioned stimulus, it is in contrast with the neutral stimulus and has a tendency to illicit a response from the subject even without the aegis of conditioning. In relation to an unconditioned stimulus, an unconditioned response has pretty much the same meaning except that it is a response, which is an innately emerging one rather than being catalyzed with the support of any sort of conditioning or learning.