MD, or Doctor of Medicine, is one of the physician designations (like DO) that identifies a physician who has completed medical school and appropriate advanced training. MDs complete four years of medical school and at least three additional years of residency training and then may be certified as specialists in such fields as family medicine, emergency medicine, pediatrics, orthopedics and so forth.
DO, or Doctor of Osteopathy, is fully trained in all aspects of medicine, just like an MD. DO's receive additional training in structural medicine, and are certified as well to do manipulation. They take a holistic approach to medicine, looking at the whole patient to determine the best treatment plan. DO's, like MD's, complete at least three years of residency and obtain certification in all medical specialties including family medicine, emergency medicine, pediatrics, orthopedics and so forth.
PA, or Physician Assistant, is a health care professional licensed to practice medicine with physician supervision. As part of their comprehensive responsibilities, PA's conduct physical exams, diagnose and treat illnesses, order and interpret tests, counsel on preventive health care, assist in surgery and in virtually all states can write prescriptions. Within the physician-PA relationship, physician assistants exercise autonomy in medical decision making and provide a broad range of diagnostic and therapeutic services. A PA's practice may also include education, research and administrative services.
NP, or Nurse Practitioner, designates a Registered Nurse with advanced education and training, who practices in all specialty areas to assess, diagnose, and treat patients of all ages and with all types of health needs. NPs have a minimum of a Masters Degree in Nursing and National Certification to practice in an advanced role.