An obstetrician is a physician who has successfully completed specialized education and training in the management of pregnancy, labour, and pueperium (the time-period directly following childbirth).
A gynecologist is a physician who has a successfully completed specialized education and training in the health of the female reproductive system, including the diagnosis and treatment of disorders and diseases.
Typically, the education and training for both fields occurs concurrently. Thus, an obstetrician/gynecologist (OB/GYN) is a physician specialist who provides medical and surgical care to women and has particular expertise in pregnancy, childbirth, and disorders of the reproductive system. This includes preventative care, prenatal care, Pap test (a test for cancer of the cervix), detection of sexually transmitted diseases, screening, and family planning.