Socialization. 
This is something most beginner dog people do not understand well.  Socializing your puppy in puppy class is not about letting the puppies all play together or even letting everyone in the class handle your puppy.  It is not about letting your puppy be petted by every Tom, Dick and Harry when you go out in public.  Socializing a puppy means taking your dog everywhere you can to expose him or her to a variety of situations both with and without dogs present.  It is about getting your puppy to focus on YOU in the face of distractions.  It takes time.  It takes effort.  It takes consistancy. 

The time frame when a dog really soaks up socialization is very short.  This period is from about 10 weeks old to 16 weeks old.  In that time you want to expose your puppy to all kinds of things in a non threatening manner.  She should see other dogs and people.. walk on different surfaces.. see puddles and water and learn that these things are not dangerous.

Socialization is about building confidence in your dog.  It is about building trust between you as the dog handler and the dog. 

After this time frame you can still socialize your dog (and you should!) but it may take more effort.  The worst thing you can do is to keep your puppy at home and never take him or her anywhere. 

There is some concern about vaccination and disease.  A puppy does not have immunity from vaccinations until 16 weeks old, when the last set of shots are given.  At that age the Mother's antibodies should have cleared the puppy's system and the puppy's own immune system should be fully developed and capable of mounting its own immune response to the vaccines.  It is important to not take your puppy to dog parks or places that have dogs that may be unvaccinated until 16 weeks old.  Talk to your vet about this.  Vaccinations is not a reason to NOT socialize your puppy in spite of the timing conflicts!

To that end, Questa was in puppy class when she was around 12 weeks old (class required all puppies be vaccinated and healthy).  I took her all over the place with me.  She learned to ride nicely in the car or truck.  She rode on the seat and I put a harness on her and attached it to the seat belt so she would not go flying if we were in an accident.  She also rode in a crate in the back seat of the car. 

She went into stores (Pet Smart is a great place to train after a puppy is 16 weeks old!) and she actually has a "fan club" in the local towns.  I took her to get soft ice cream (vanilla.. chocolate is poisonous to dogs).  When she is eating her ice cream (taught her to politely lick) she is totally focused on the ice cream experience.  I believe a bomb could go off next to her and she would not notice.  I took her in the Feed Stores and Tractor Supply.  Since I live in a rural area, I took her for walks in the city and on bike paths.  She walked on board walks and grass and concrete and gravel.  She walked on plastic and through puddles.  I showed her the creek and she got to go wading (on a line). 

She went to the Nursing home as soon as she had her Rabies shot to visit my Dad. 

She learned to walk on a leash.  She learned to lie down.  She learned to come when called.  She learned to sit.  She learned that it is not acceptable to run up to other people and dogs (we work on that one to this day.. some lessons are harder than others). 

I still take Questa with me when I go places.. her fans ask for her!  She is usually well mannered and rides well in the car.  It took effort and time to get this, but it was well worth it.  The result is that my dog is welcome most places we go.