Step-by-step explanation:
Primary care physician is a doctor who treats general illness and injuries. A specialist is the doctor who is specialized in one field of medicine.
Primary care physician can treat general ailments. I would go to the general physician if I have been suffering from high fever or flu or headache or minor injury from outside.
A Specialist can treat major ailments of his specialized area. For example if I am suffering from severe knee problem, or my bones are weakening, then i would visit a doctor who is specialist in the field of bones, i-e i would visit an orthopedic.