A doctor can refer to anyone holding a doctorate (PhD) but usually the term means a physician—a medical doctor (MD).
A physician is a professional who practices medicine, which is concerned with promoting, maintaining or restoring human health through the study, diagnosis, and treatment of disease, injury, and other physical and mental impairments. They may focus their practice on certain disease categories, types of patients, or methods of treatment – known as specialist medical practitioners – or assume responsibility for the provision of continuing and comprehensive medical care to individuals, families, and communities – known as general practitioners.  Medical practice properly requires both a detailed knowledge of the academic subjects (such as anatomy and physiology) underlying diseases and their treatment – the science of medicine – and also a decent competence in its applied practice – the art or craft of medicine.
Both the role of the physician and the meaning of the word itself vary around the World, including a wide variety of qualifications and degrees, but there are some common elements. For example, the ethics of medicine require that physicians show consideration, compassion and benevolence for their patients.
- World Health Organization: Classifying health workers. Geneva, 2010.
Incorporates material from Wikipedia