Primary Care is the doc you go to intially for all of your problems, the doc you go to when you feel ill, the doc you go to when your body just doesnt feel right. He/she is supposed to diagnose the "source" of the problem and if necessary transfer you to the appropirate specialist.
A primary care doc is usually a doc of internal medicine or family practice, they get generalized training in internal medicine mostly dealing specifically for internals of adult for the internal medicine doc, the family practice deals with all ages and also provides more of a psychological role. They are allowed to perform in office surical procedures like mole removal and small stiches but its really limited by the type of plan you have and what the doc has insured themselves against.
Some of them can provide specialized roles, like the doc i go to is considered an internal medicine doc but also sees patient's as an endocrinologist. It is really dependent on the doc though and what they insure themselves against, what training they have recieved, and what certifications they hold. I know some docs who have certifications for multiple roles, and others that are just certified for family practice.