A Physiologist by training, Ivan Pavlov's most famous experiment in psychology began as a study of digestion. Although very concerned with biases and controlling for variables, during his experiment he noticed that his subjects (dogs) would illicit a salivation response prior to being presented with food. He began to explore this phenomenon and eventually developed one of the most applauded behavioral studies on conditioning that won him the Nobel Prize.
Classical conditioning refers to the phenomenon of stimulus pairing that result in similar responses. In other words, when food (referred to as the unconditioned or unlearned stimulus) is paired with the sound of a bell, the sound of the bell (called the conditioned or learned stimulus) eventually results in the same response (salivation).