PA = Physicians assistant. They go through an intensive two year program, and yes they are always under doctor's supervision. They perscribe medicine but the doc will sign off on it. They can specialize as well. Generally they do the dirty work and busy work to help out the MD/DO to save them time. They are paid reasonably well and do not pay malpractice insurance, they also have good quality of life.
NP= Nurse Practitioner. These are RNs who after some years of experience go through some more extensive graduate training similar to PA school. They are mostly found in family practice, but they can also specialize in ob-gyn, er, and other settings. They do not need MD/DO supervision, and can run their own practice/clinic. Pay and quality of life is comparable to PA.
Hope this helps, maybe someone who is actually a PA or a NP can give more insight.
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