A neutral stimulus (such as a bell, or an object), that is repeatedly followed by unconditioned stimulus, eventually elicits a conditioned response. For instance, in Ivan Pavlov's famous classical conditioning study, he repeatedly paired a neutral stimulus (a ringing bell) that came before an unconditioned stimulus (food), and eventually the sound of the bell alone elicited a conditioned response (salivating) in dogs.
A neutral stimulus is a stimulus which originally gives no particular response other than concentrating recognition. In traditional conditioning, when practiced collectively with an unconditioned stimulus, the neutral stimulus converts a conditioned stimulus.
The conditioned stimulus is an earlier neutral stimulus that, after converting correlated with the unconditioned stimulus, finally appears to trigger a conditioned acknowledgment.