Summary
To summarize, classical conditioning (later developed by John Watson) involves learning to associate an unconditioned stimulus that already brings about a particular response (i.e. a reflex) with a new (conditioned) stimulus, so that the new stimulus brings about the same response.
pavlov classical conditioning diagram
Pavlov developed some rather unfriendly technical terms to describe this process. The unconditioned stimulus (or UCS) is the object or event that originally produces the reflexive / natural response.
The response to this is called the unconditioned response (or UCR). The neutral stimulus (NS) is a new stimulus that does not produce a response.
Once the neutral stimulus has become associated with the unconditioned stimulus, it becomes a conditioned stimulus (CS). The conditioned response (CR) is the response to the conditioned stimulus.