Well the NRMP match is kinda complicated, but let's take a look at the scenario.
Fourth-year med student decides that he'd like to go into general surgery, and so prepares his NRMP stuff for a career in general surgery. He interviews at various programs, and on 2/16/00, submits his rank order lists (his personal ranking of programs where he's been interviewed), and waits patiently for 3/16/00, the Match Day. On the Monday of the Match Day week, he'll get a call from the Dean of the Medical School telling him that he did NOT match to any of the programs on his rank order list. The student must now SCRAMBLE to find an open spot, whether it's in an IM program, Peds, FP, or even a General Surgery spot. Can he theoretically still continue his intended career as a General Surgeon? I guess so, but here's the problem.
Most competitive programs have filled up all their categorical spots, and what's left over are preliminary spots (which are only one year long) in non-competitive programs. Should the student decide that he'll go for a prelim spot, he'll be a surgical resident for one-year, after which he must find a program that will take him into a PGY2 (second-year of residency training) spot for the continuation of his training, or apply for other types of programs that start PGY2 (EM, Optho, PM&R, Derm, Uro, etc.). Generally this is very difficult to do if you didn't already match into those programs as a med student.
Not matching is kinda bad because you eventually end up in some place you never thoguht you'd end up, taking on work you didn't see yourself doing just a year ago.
I know one person who applied to Derm but didn't match. She then SCRAMBLED and found a prelim spot in IM at a great hospital. In some sense she's lucky to have found a good program at a great hospital, but she still didn't match what she wanted.
You have NO guarantee that you'll become the kind of doctor you want when you enter medical school. Tons of students apply to Ortho programs throught he NRMP match every year, yet a large percentage fail to match. Their options are 1) do something else (see above), 2) go into a prelim surgical spot and reapply to ortho programs, which start PGY2 anyway. BUT reapplication doesn't necessarily guarantee your acceptance. I know a guy who did exactly that last year: fail to match Ortho, went into a prelim program, tried for Ortho again, and failed to match a second time. He was lucky enough, however, to have been offered a spot in the surgical program where he'd been for the past year, so he's continuing life as a general surgeon.
Hope I haven't confused you too much. It's a complicated process -- almost as complicated as renal physiology -- but it works out for all involved.
Tim of New York City.