A simple surgical procedure allowed Pavlov to measure accurately the amount of saliva secreted by a dog. When Pavlov presented the dog with a piece of meat, the dog exhibited a noticeable increase in salivation. When Pavlov withheld the presentation of meat and nearly rang a bell, the dog has no salivation. The Pavlov proceeded to link the meat and the ringing of the bell. After repeatedly hearing the bell before getting the food, the dog began to salivate nearly at the sound of the bell even if no food was offered. In effect, the dog learned to respond that is to salivate the bell.
The meat was unconditioned stimuli. It invariably caused to dog the react in a specific way. The reactions that took place, whenever; the bell was the artificial stimulus or conditioned stimulus. While it was originally neutral, when the bell was paired with the meat (an unconditioned stimulus), if eventually produced a response when presented alone. It is the conditioned response. This describes the behavior of the dog salivating in reaction to the bell alone.
Using these concepts, we can summarize the classical conditioning. Learning a conditioned response involve building up an association between a conditioned stimulus and an unconditioned stimulus.
Using the paired stimulus, one compelling and other one neutral, the neutral one becomes a conditioned stimulus and hence takes on the properties of the unconditioned stimulus.