During the 1890s, Ivan Pavlov was a Russian physiologist who was researching salivation in dogs as a response to being fed. While the dogs were being fed, he used a small test tube to measure the saliva by inserting it into their cheeks. He predicted that the dogs would salivate when the food was in front of them, but he realized that the salivation actually started when the dogs heard the footsteps of his assistant. The salivation was not only a result of the food being served, but of the sound of the footsteps that the dogs associated with being fed. This is how classical conditioning was discovered.
Pavlov had the idea that dogs do not need to learn certain things, such as salivating when they see food. He said these reflexes are hard-wired into dogs. He coined the dog’s food as an unconditioned stimulus and the salivation as an unconditioned response - this response did not require any learning on the dog’s part.
Unconditioned Stimulus (Food) > Unconditioned Response (Salivate)
He then began working with neutral stimuli (things that did not elicit responses from dogs instinctively) and he wanted to “condition” the dogs. This means he wanted to elicit a response each time a new stimuli occurred. He used a metronome and clicked it right before giving the dogs food. Over time, the sounds of the clicking metronome caused an increase in salivation.
Conditioned Stimulus (Metronome) > Conditioned Response (Salivate)
This means that the dog learned to associate the metronome and the food with a learned behavior. This response was learned (conditioned), which was referred to as a conditioned response or a Pavlovian response.
It was founded that in order for associations between two stimuli to be made (such as the metronome and the food), they had to be presented close together in time. He said that if the time between the two items were too long, the learning would not occur. His work became famous, largely because it was the first systematic study of the basic laws of learning and conditioning.
To summarize: classical conditioning - learning to associate an unconditioned stimulus (food) that already brings about a particular response (salivating) with a new conditioned stimulus (metronome), so that the new stimulus brings about the same response. Meaning, when the metronome sounded, the dog would salivate, even without the food being present.
References
Pavlov, I. P. (1897/1902). The work of the digestive glands. London: Griffin.
Pavlov, I. P. (1928). Lectures on conditioned reflexes. (Translated by W.H. Gantt) London: Allen and Unwin.
Pavlov, I.P. (1927). Conditioned Reflexes: An investigation of the physiological activity of the cerebral cortex. Retrieved from http://psychclassics.yorku.ca/Pavlov/lecture6.htm.