In general, the word "doctor" refers to any medical professional who is licensed to practice medicine. A medical doctor, also called an M.D., is a person who has graduated from an accredited four-year medical school that meets the standards of the American Association of Medical Colleges and is licensed and certified to practice medicine.
Medical doctors can practice a wide array of specialties, including pediatrics, obstetrics and gynecology, neurology, dermatology, and internal medicine. Primary care physicians are usually internists or family practitioners (though some PCPs can have different specialties) who focus on building relationships with patients over time.