After interviewing and applying to hospitals, applicants rank the hospitals where they want to be residents. The programs you desperately want to work at go at the top. Those you like but aren't quite as interested in go farther down the list. Don't put a program you're not sure you'll be comfortable at on your list, because if you're matched to that program, you're obligated to attend it. No takebacks.
What does it mean to be matched to a program? After all the interviewing and applying, and after all the applicants rank their programs, the programs then rank their applicants. A computer algorithm compares all the lists, and then matches applicants with residency programs. In mid-March, all applicants are notified if they were matched or not. Match Day, the day when all medical students nervously open envelopes to see where they were matched, follows on the third Friday in March. Are they headed to prestigious programs in their own hometowns? Or to programs they ranked lowest five states away?
What if you don't get a match at all? Then you get to participate in "the scramble," which basically means making a lot of phone calls to residency programs that still have openings. Starting in 2012, the National Residency Matching Program (NRMP) will conduct an organized scramble. Unfilled positions will be offered through ERAS, with applicants having a time limit of two to three hours to accept or decline the offer. This is called the Supplemental Offer and Acceptance Program (SOAP). If that doesn't work, you'll have to start the entire process again the next year.
There are other matching programs in the U.S., including one for military hospitals and one for osteopathic medicine. They generally hold Match Day earlier than the NRMP's Match Day so that unmatched applicants can try for an NRMP match.