MD = Medical Doctor
PA = Physician's Assistant - an "almost Dr" - they can do a LOT of medical procedures as long as they are under the (very) loose control of an MD - if I remeber right, it can be by phone in many cases
NP = Nurse Practitioner -Slightly lower down the scale - Don't remember exactly what their rights are
RN = Registered Nurse - aka, a REAL nurse - gone through REAL schooling. These folks are the real front line troops in the average hospital

PAs and NPs were an outgrowth of the MD shortage of the 50s, 60s, 70s etc. They fill the nitch between a nurse, and an MD. The first time _I_ heard the concept was in a 1940s article that proposed the idea of a "Nursing Dr"