Most* Nurse practitioners (NP), also called Advanced Practice Nurses (APN) have completed a registered nursing degree as a part of a bachelors degree, plus a masters degree. In addition, many now seek a doctorate level degree.
(*Not all NPs do have these degrees. In some states, NPs were licensed prior to these degree requirements, and were therefore "grandfathered," meaning, they did not have to complete them.)
An NP may be certified in a specialty area, such as family health, oncology, or pediatrics.
Nurse practitioners are clinicians, not unlike a physician who may have his or her own practice. He or she can serve as a patient’s regular healthcare provider, and may diagnose, order tests, develop treatment plans and write prescriptions.
Most NPs work in collaboration with a physician, which is usually required by the state they work in. They are accredited through several organizations, including the American Academy of Nurse Practitioners.
Be sure you don't confuse nurse practitioners with nurses. Their work with patients may be very different.
Because the numbers of primary care physicians are decreasing, and because healthcare reform will create even larger demand for primary care clinicians, it is expected that the numbers of nurse practitioners will grow very fast through this decade to increase the numbers of clinicians who offer primary care.
Two terms are sometimes used to describe nurse practitioners that are opposed by the Academy of Nurse Practitioners. Those terms are "physician extender" and "mid-level provider." The Academy feels as if both terms are imposed on them by heathcare groups who have a stake in making NPs seem like "less" of clinicians than physicians, or unable to offer the high level of care offered by others with different types of training. Neither case is true.