Classical conditioning - (also Pavlovian conditioning or respondent conditioning) is a form of learning in which the conditioned stimulus or CS, comes to signal the occurrence of a second stimulus, the unconditioned stimulus or US. (A stimulus is a factor that causes a response in an organism.) The conditioned response is the learned response to the previously neutral stimulus.1 The US is usually a biologically significant stimulus such as food or pain that elicits a response from the start; this is called the unconditioned response or UR. The CS usually produces no particular response at first, but after conditioning it elicits the conditioned response or CR. Classical conditioning differs from operant or instrumental conditioning, in which behavior emitted by the subject is strengthened or weakened by its consequences (i.e. reward or punishment).2
There are many examples of these innate (unconditioned) reflexes (corneal, coughing, swallowing, withdrawal reflexes etc.) or learned (conditioned) reflexes.3:p. 82
"In any animal, regardless of its prior history, painful stimulation of the foot causes the leg to be withdrawn by bending at all its joints. This flexor reflex is an example of an unconditioned reflex, an innate response based on fixed connections in the chain of neurons from the receptor (sensor) to the effector. Of still more interest in everyday life are the acquired or conditioned reflexes, in which the functional connections between the excited sensors and the patterns of activity in effector organs become established by learning process."4:p. 155
Conditioning is usually done by pairing the two stimuli, as in Pavlovâ€™s classic experiments.5 Pavlov presented dogs with a ringing bell followed by food. The food elicited salivation (UR), and after repeated bell-food pairings the bell also caused the dogs to salivate. In this experiment, the unconditioned stimulus is the dog food as it produces an unconditioned response, saliva. The conditioned stimulus is the ringing bell and it produces a conditioned response of the dogs producing saliva.
It was originally thought that the process underlying classical conditioning was one where the conditioned stimulus becomes associated with, and eventually elicits, the unconditioned response. But many observations do not support this hypothesis. For example, the conditioned response is often quite different from the unconditioned response. Learning theorists now more commonly suggest that the CS comes to signal or predict the US. In the case of the salivating dogs in Pavlov's experiment, the bell tone signaled and predicted the arrival of the dog food, thus resulting in the dog salivating.6 Robert A. Rescorla provided a clear summary of this change in thinking, and its consequences, in his 1988 article "Pavlovian conditioning: It's not what you think it is." Wikipedia, Conditioning
Unconditioned Reflex - Normally dogs snatch at food; elephants reach out their trunks for it. A man will pull his hand away from a hot plate. These are examples of unconditioned reflexes, fully developed in infancy. They constitute the equipment with which the animal faces life, according to the behaviorists.* By modifying the conditions, the simple reflexes may be changed, becoming more complicated, or conditioned. The process of changing an unconditioned reflex into a conditioned reflex was clearly demonstrated to an audience of psychiatrists at the Academy of Medicine last week, in a cinema entitled "The Mechanics of the Brain." http://www.time.com/time/magazine/article/0,9171,786747,00.html