Classical conditioning (also Pavlovian conditioning) is a form of associative learning.
Ivan Pavlov was the first to show in what way it works. He did this in 1927, using dogs. There is one stimulus which is called neutral, and there is another, which has some meaning. If the two stimuli are often presented together, the organism learns that they belong together. As a result, it is enough to show the neutral stimulus to get what Pavlov referred this learned relationships to as a conditional reflex, or to what is today called a conditioned reflex or response. ....
Pavlov did the experiment with dogs. Each time before he fed the dogs, he rang a bell. The dogs then learned that when the bell rang they would be fed. So they started to salivate, when they heard the bell ring, even before they saw or smelled the food.
- Pavlov I.P.  1960. Conditional reflexes. New York: Dover Publications. The 1960 edition is an unaltered republication of the 1927 translation by Oxford University Press.