What is the difference between a nurse practitioner and a physicians assistant?

From a practical standpoint, the difference is a legal one. Both are required to practice under the oversight of a physician (at least in most states, if not all) but the nurse practitioner is required to execute pre-provided protocols, created by a physician or group thereof; the physician's assistant has somewhat greater autonomy.

From a practical standpoint, as a physician that has had to work as a supervisor both to advance practice nurses (which includes nurse practitioners) and physicians assistants, the difference is nearly nil: both end up doing whatever the supervising physician trusts them to do, with as much (or as little) autonomy as the physician is comfortable extending to them. I've known some in both groups that functioned almost on a par with a physician. I've known a few that thought they were on a par with a physician, and that weren't, by a very long ways.

In the long haul, the real difference isn't the label, it's the experience, wisdom and integrity of the person behind the label that counts.

Nurse practitioners (NPs) are registered nurses with advanced training in nursing. They specialize during their training, such as a pediatric NP, geriatric NP, family health NP, or women's health NP. Physician assistants are educated in the broad spectrum of medical and surgical care, allowing them to practice in the variety of specialties rather than one specific area of study.

The education for a NP is degree based -- that is, nursing students are required to obtain a specific degree in an advanced nursing specialty as a requirement to practice as a NP. Physician assistant education is competency based -- that is, they must master certain clinical skills taught by an accredited PA program.

Another difference between NPs and PAs is their relationship to physicians. PAs work with physicians in a team model, with the physician providing appropriate supervision to ensure the patient is receiving quality medical care. This is because PAs and physicians both practice medicine.

NPs, in most states, work in a collaborative model with physicians, that is having a physician to whom the NP can refer patients requiring specific medical care. This is because NPs practice nursing under the state nursing regulations rather than providing medical care under the state medical practice act.

An NP is a high ranking nurse. In practice, they can specialize and the job seems very satisfactory and independant.
A PA is like a doctor's ******... if you are going to go to school, be an NP.