Pavlov is at work with your puppy

Classical conditioning is defined as conditioning in which the conditioned stimulus is paired with and precedes the unconditioned stimulus until the conditioned stimulus alone is sufficient to elicit the response.  This phrase was coined by Ivan Pavlov and is always at work in dog training. 

It is incredibly important to realize that while dogs do learn by operant conditioning, classical conditioning is working too.  Dogs can associate objects or people as good or bad.  For instance, if everytime your dog is around children your dog gets kicked in the face the dog learns to associate children (neutral stimulus)= kick in the face (bad thing).  Eventually, this dog may see the situation like this: children are a bad thing.  The children are no longer a neutral stimulus and worse they are a predictor of bad things.

If everytime your dog is around children and they drop food on the floor (as children often do) then dog learns that children=yummy food on the floor (good thing).  Overtime dogs learn that children are a good thing.

Let’s look at another example.  When Rufus the terrier was a puppy, he saw a man a couple times with a beard that harassed him.  This bearded man poked him, made him bark and laughed when he tried to run away.  Now when Rufus sees a bearded man- he thinks that they are going to harass him.  The bearded man (which was a neutral stimulus) has become a predictor of bad things to come (a conditioned stimulus)

If Rufus’s experience were different where everytime he met bearded man he got a really yummy cookie, the bearded man (which was a neutral stimulus) would be a predictor of good things (a conditioned stimulus).

For some dogs it takes many bad experiences to react to a situation and for others it only takes one.  When you are socializing a puppy, you are working to make positive associations with new and novel people, objects, etc. 

Classical conditioning does not work alone.  Operant conditioning is always at working the background too which makes it sometimes confusing as what to reward and what not to reward.