Classical (Pavlovian) conditioning is simply the pairing of a conditional and unconditional stimulus to a particular response.

One of the best examples of this classical conditioning was brought to my attention when a client, whose dog I was treating for nervous aggression towards visitors arriving at the house, called me on the telephone for a weekly progress report.

Dusty, a two year old Labrador, had always displayed aggressive behaviour when anyone other than his owners arrived at the house.  His owners brought Dusty along to see me in the hope that we could extinguish the unwanted behaviour and so, as Dusty really liked his food, I advised a four week preliminary programme to change his response to people arriving at the door (counter conditioning).

By the third week of the programme the owner phoned to say that Dusty had now become accustomed to being fed whenever anyone outside the immediate family entered the house.  His previous aggressive display had almost disappeared but now the owners had observed that Dusty began to drool profusely whenever the front doorbell rang!

The video below serves to indicate how Dusty had been classically conditioned to salivate whenever the doorbell rang, because he had paired the two events and therefore used the sound of the bell as a reliable predictor of the subsequent presentation of food.

In this model the dog learns that the door bell being rung is a reliable indicator of the subsequent presentation of food.

It should be noted that no new behaviours have been learned (salivation is a naturally occurring function) all that has happened is that Dusty had learned to pair the previously different functional stimulus (the previously neutral stimulus) of the doorbell to the production of food.

Dusty has not been trained to salivate as he does this automatically without any previous learning experience. 

Further reading

Stimulus Response Theory