Classical Conditioning is form of behavioural learning that has an impact on how we learn, behave and respond according to different influences and incidents that has happened to us overtime or during one bigger and more memorable occurrence.
Usually classical conditioning is a learned behaviour through paired association over time. A repeated response to a repeated action, thing or sound. In some cases, though you can be conditioned to respond to something in the same or similar manner, to something that initiated a strong natural response at the time. A more traumatic and in turn more memorable incident. Such as an car accident and then being nervous around vehicles. Until the accident your fine around cars and have no reason to feel nervous or scared. After it becomes a learned or conditioned response, as you have a cause and reason to feel and react in such a way.
Classical Conditioning is can be separated into 5 different components.
The UCS , UCR,NS,CS and CR.
The UCS is the unconditioned stimulus, essentially that means something that has no prior response from you. You may know what the stimulus is, and use it for its intended purpose. Or see/hear it on a regular basis and have no reason to associate it with anything other than what it is.
The UCR is the unconditioned response. Your natural instinctive response to something.
The NS is the Neutral Stimulus. which Is also an unconditioned or unassociated thing that does not initially cause a reaction.
The CS is the Conditioned Stimulus. This is the previously Neutral Stimulus that now elicits a conditioned or taught response.
The CR is the Conditioned response to the Conditioned Stimulus. This means that you now respond in a conditioned or learned manner to the prior Unconditioned Stimulus.