Classical conditioning is a way to produce a learned association between two things in the environment. It first relies on the automatic response to a stimulus, and then presents a new stimulus before the unconditioned stimulus. The point is to condition this new stimulus to produce the same response that the unconditioned stimulus already produces. Classical conditioning includes an unconditioned stimulus which creates an unconditioned response. This is the natural stimulus and natural response. Then to create the desired behavior which is the conditioned response, the conditioned stimulus (the desired stimulus to create the same response as the unconditioned stimulus), is presented before the unconditioned stimulus. The conditioned stimulus is formerly the neutral stimulus because it does not produce a behavior.
When my dog was a puppy, there was no way to get her to come inside before she knew her name or any commands. Originally, if we would just call her name, there would be no reaction. To train here to come inside, we would yell her name and then shake her treats. When she came, we would give her a treat. After a while, she would just come inside when her name was called. This helped her learn her name as well. We would still give her treats every now and then to reinforce the behavior and to avoid extinction.
In this case, the unconditioned stimulus would be the treats, and the unconditioned response would be running to them. The conditioned stimulus would be yelling her name, and the conditioned response would be running to the door when her name is called. The neutral stimulus would be calling her name, because it originally did not elicit a response. When using classical conditioning, it is crucial to present the conditioned stimulus before the unconditioned stimulus. This is why we would call her name before shaking the treats.