Learning is a relatively permanent change in an organisms behavior due to experience. What is learnable we can potentially teach. Experience is key to learning. Through associative learning we come to link certain events. The process of learning association between events is called conditioning.
Associative learning had been discussed for centuries, it remained, for Ivan Pavlov, to capture the phenomenon of classical conditioning. Pavlov repeatedly presented a neutral stimulus, just before an unconditioned stimulus, triggered an unconditioned response. After several repetitions, the stimulus alone began triggering a conditioned response. Further experiments on acquisition revealed that classical conditioning was usually greatest when the CS was presented just before the UCS, thus preparing the organism for what was comming.
The first stage in response learning involves the association of the CS with the US. Responses are subsequently weakened if they are not reinforced, but they may reappear after a pause, response may be triggered by stimuli similar to the conditioned stimulus, but not by a dissimilar stimuli.
The behaviorists optimism that learning principles would generalize from one response to another and from one species to another has been tempered. Conditioning principles we know are cognitively and biologically constrained. In classical conditioning, animals learn when to expect an unconditioned stimulus. Moreover biological predispositions make learning some associations easier than learning others. For example Rats are biologically predisposed to learn associations between a peculiar taste and a sickness. They will avoid the drink with that taste. They do not learn to avoid a sickening drink announced by a noise.
Pavlov's conditioning principles generalize to many species and are applicable to humans, especially to learning of emotions, such as fear. His objective methods illustrated how psychology could proceed as a science.