A doctor is a healthcare worker who diagnoses and treat human diseases, injuries or other ailments. Doctors can specialize in one type of medicine or in the treatment of different type of body parts. So you get specialists such as dentist (for the teeth), cardiologist (for the heart), Gastroenterologist (diagnose and treat conditions and diseases of the gastrointestinal tract) and so on. A general practitioner (GP) is not a specialist. He or she treats common conditions and refer a patient to a hospital or specialist for specialist and urgent care.