This is learning by association. A Russian physiologist called Ivan Pavlov, studied salivation in dogs as part of his research programme. Normally, dogs will salivate at the when food is presented, but Pavlov was interested why the dogs had started to salivate when the saw the people that usually fed them (they also responded to the sound of the dishes being used for their meals). Pavlov set up an experiment to find out if the dogs could be trained to salivate at other stimuli such as the sound of a bell or a light. At feeding times, Pavlov would ring a bell and the amount of saliva produced by the dog was measured. After several 'trials' Pavlov rang the bell without presenting the food and found that the dogs salivated in the same way as if food was being presented.
You will note that the conditional response is the same as the unconditioned response, the only difference being that the response is evoked by a different stimulus.