Overview of Physician Extenders:
Nurse Practitioners (NPs):
Nurse Practitioners have received advanced education in their fields which may be focused on primary care, or dozens of specialties. They may practice on their own, diagnosing and treating patients, or they may work alongside a physician in a hospital or other type of practice.
You can find a more extensive description of nurse practitioners here.
You'll want to understand the differences between nurses and nurse practitioners.
You'll also want to understand why some nurse practitioners object to the use of the term "physician extender" when used to describe them.
Physician Assistants (PAs):
PAs are licensed to practice medicine under the supervision of private practice physicians or hospitals. They may diagnose and treat patients and prescribe medicine, and some may be surgical assistants.
Physicians assistants are relatively new to the American healthcare landscape, with roots in the post-Vietnam era when there was a physician shortage in the United States. Their education requires a bachelor's degree, plus two to three years of additional training that results in a relevant master's degree.
PAs are accredited by the Accreditation Review Commission on Education for the Physician Assistant (ARC-PA).

