A dog's critical development period is from 3 to 16 weeks.  It is our job as an owner of a new puppy to make sure that we continue the socialization process especially during this most important time and continuing as your dog matures. 

To do this we introduce our pups to nice, gentle people and friendly dogs outside of our immediate family.  Positive interaction is a must here.  First meetings of all sorts will determine how your dog feels and remembers these instances.  Your puppy needs to meet children, elderly people, people of different races and ethnicities, people in wheelchairs, baby strollers, nice older dogs of other breeds, and friendly puppies.  They also need to be introduced to tile floors, carpeting, and concrete, ride in the car, go to Home Depot, strip malls, outdoor patios (if dogs are allowed), and to friends houses, etc.

The shelters are full of dogs that are dumped for behaviors that are exhibited due to lack of socialization and proper training.  This is the fault of the owners that did not do their part.  The dog only knows what we show and teach them.  This is not the fault of the dogs, but they are the ones that pay the ultimate price.

In order for our dogs to be confident, well behaved, and to mature to their full potential, it's up to us as their guardians to ensure their success.  A good Puppy Class that offers off leash play and interaction with other nice people is the most important class that your dog can attend. 

 Here is what the American Veterinary Society of Animal Behavior has to say about early socialization for puppies.....

http://www.avsabonline.org/avsabonline/images/stories/Position_Statements/puppy%20socialization.pdf