MEDICAL STUDENT - Someone who has received a bachelor's degree and entered a four-year graduate program (medical school).  Usually, the first two years of medical school are spent in the classroom, and the final two years are spent in a clinical setting where the student can experience more hands-on learning.  A medical student is a doctor in training.  A license to practice medicine requires graduating from medical school and an additional year as an intern in a teaching hospital.  John Carter was a third-year medical student in the show's first season.  He returned the second season as a fourth-year medical student doing a surgical sub-internship (sub-I), which is a more intensive training program that functions something like an audition prior to entering an internship.  Lucy Knight was a fourth-year medical student in season five. 

RESIDENT - The hospital residency program begins with the rank of second-year resident and involves specialized training in areas such as surgery, pediatrics, internal medicine and psychiatry.  Residencies last from two to six years, depending on the specialty.  The first season, Dr. Peter Benton was a second-year surgical resident; Dr. Susan Lewis was a second-year resident in emergency medicine; and Dr. Mark Greene was a fourth-year emergency medicine resident and chief resident of the emergency room.  In season two, Benton and Lewis returned as third-year residents; Greene was promoted to attending physician.  In season three, Lewis and Benton continued as fourth-year residents.  In season four, Benton was a fifth-year resident. 

CHIEF RESIDENT - A doctor in his or her senior year of residency who assumes responsibility for overseeing interns and junior residents.  The chief resident is selected by attendings, and the title is considered a prestigious honor.  Dr. Mark Greene was chief resident of the emergency room in season one; Dr. Kerry Weaver was chief resident of the emergency room in season two. 

ATTENDING PHYSICIAN - Someone who has completed all medical training and received privileges (that is, they are allowed to practice medicine) at a hospital.  An attending physician is also responsible for supervising residents and medical students.  The buck--professionally, legally and ethically--stops here.  Dr. Greene became an attending physician in the second season of "ER"; Dr. Weaver became an attending in season three.  They both continued to hold those positions until the end of season four.  In season five, Dr. Ross moved up to the position of pediatric emergency attending physician. 

PHYSICIAN ASSISTANT (PA) - Someone who has completed a PA training course of two years in addition to at least two years of college.  Physician assistants provide patient care under the supervision of a licensed physician, performing physical exams and diagnosing and treating illnesses.  Jeanie Boulet was a licensed physician assistant. 

REGISTERED NURSE (RN) - Someone who has attended nursing school, typically for four years, and received a degree in nursing.  Registered nurses provide care to patients based on orders written by doctors.  Carol Hathaway is a registered nurse with an advanced master's degree in nursing, qualifying her to be a nurse manager in charge of the other nurses.

PARAMEDIC - Individuals, often firefighters, trained in advanced life support, who transport patients by ambulance to the emergency room.  Paramedics may give certain medications en route to the hospital with approval from a physician.