This article is from the ER FAQ, by Rose Cooper firstname.lastname@example.org with numerous contributions by others.
*As we near the middle of season five, this is the lineup, generally
-Lucy Knight, the New Kid On The Block, is a third-year medical
student, pretty much like Carter was when he first arrived on the
scene at County. [Contrary to the funny, but erroneous,
Young Female Doctor Void-filler tag bestowed upon the actress
playing her, the Knight character isn't yet a doctor. Though that
mever stopped anyone referring to Young Carter thusly...]
-John Carter and Maggie Doyle are both third-year residents; however,
as noted above, Carter is only a second-year ER resident, so Doyle
outranks him [but since Doyle will likely continue to be doyled, you
wouldn't know it from watching...]
-Peter Benton is in the sixth year of his surgical residency
*(Elizabeth Corday's training made her the equivalent but in season
five, she majically became an intern again. This is the part
-Mark Greene was a fourth-year resident in the first season of the
show; now, he is finished with his residency, and works as an
attending, as does Kerry Weaver [who, by her own hand, resigned first
as *ACTING* Chief, then as -Interim- Chief. But that's Another
Story]. Most of the "upper-level" doctors seen on the show, like
Hicks, Kaysen, Vucelich, etc., are also attending physicians.
-Prior to season five, Doug Ross was a Pediatric Fellow--that is to say,
he is finished with his residency (which is traditionally two years
long), but instead of working as an attending (at that point), his
position with the hospital was there because of a fellowship grant.
*In season five, Doug gave up his Fellowship and became a Pediatric
See FAQ Section 6 ("Medical Questions") for detailed info on the ER hierarchy.