I believe a primary care physicians are ones who are trained to look for basic health problems. So, if you have a concern about chest pains, you would inform them about having them for lets say the past two days.
A specialist is someone who looks more specifically into the health problem. I used chest pain above, so you go to a specialist to see if you are having heart problems.
Both are able to help treat people of illnesses and are able to save lives if it was needed.
Hopefully this helped.