Most fourth-year medical students are eagerly awaiting noon on Monday of Match week to find out if they matched. If they didn't match or matched to a second-year position but not first-year, these is a process called the "Scramble" that they will be taking part in if they do not have a PGY-1 position. The majority of US medical school seniors will match if the process has worked well. By process, I mean that they have ranked places that interviewed them and the places that interviewed them, have ranked them. The computer attempts to match the applicants highest choice with the place that ranked them the highest. Most of the time, the process works out fine but sometimes it doesn't and an applicant will know on Monday if they need to actively look for a position.
There will be a scramble list published on noon of Tuesday that will include programs that didn't fill all of their PGY-1 slots. Some programs will elect to offer positions to people who applied outside of the match. This means that you need to be aware that many programs that you might have applied to will fill from people who have either applied and are eligible to accept a position outside of the match which means that those programs may show open slots but those slots are filled (usually by FMGs who have already signed contracts). The only way to find out that a slot on the scramble list is truly open is to call the program on scramble day and find an open slot. You also need to have your application materials ready to fax/send by e-mail thus you would need to be next to a fax machine or have your materials in electronic form and ready to send if requested.
On scramble day, programs that didn't fill (and want to fill) will have the program director and a couple of faculty, reviewing applications received and ready to make an offer of a contract on the spot. This is why is makes sense to be at your medical school (most schools will have a scramble set up) and have your materials ready to send. On scramble day, some programs that show openings will be literally deluged by unsolicited faxed applications from commercial services for applicants who didn't match. Most of those folks will receive an e-mail that stated that the program will not review unsolicited applications. Some unmatched applicants will spend literally hundreds of dollars with commercial services that will sent applications but be aware that unless you have spoken to a program, they are not likely to review your application.
Many very high-powered and excellent residency programs may not fill for a variety of reasons from clerical errors to not interviewing enough candidates. Every year some places that have national reputations may have openings. Most years, the preliminary spots at many programs will not fill completely because not enough people applied for those slots. This means that a US grad (who is at their medical school on scramble day) stands a good chance of getting into one of those unfilled preliminary spots. Many programs will call deans looking for unmatched candidates for slots which is a great way to snag an open position at a top-ranked program.
The scramble situation is very stressful for those who go through it but the worst case scenario is that an applicant spends one year in a preliminary position and enters the match as stronger candidate the next year having done one year of excellent work. If a candidate accepts a preliminary position, they have to work very hard to be sure that they get good letters and good experience to take into the Match for the next year. Most good program directors will help a preliminary candidate find a categorical position for the next year if they have done an outstanding job with the preliminary year. In short, doing a prelim year isn't the end of the world and can provide valuable time for making good contacts with programs that interest you for the next year. If you know that you will be going into the Match after a preliminary year, be prepared to make sure that your letters and application materials are sent to your medical school for uploading as early as possible. You don't want to risk not matching a second time.
My other piece of advice for anyone reading this post who receives a letter on Monday that says they are going to scramble, is that you need to immediately contact your dean of students at your medical school. You don't want to try to go through the scramble from a remote location without having the advantage of experienced people to help you through this. You need to learn as much about the process as you can in the 24 hours before the scramble starts. Positions, even the preliminary ones, will go very quickly. If you find that you don't have a PGY-1 position after the first 24 hours of the scramble, you are going to need to contact your medical school to let them know. Again, programs that don't fill (or who have open positions) will contact the deans at medical schools looking for candidates first. You want to be sure to let your school administration know that you are looking for a job.
To those that match and don't make the mistake of thinking that you are inferior because you didn't get your first or second choice. If you didn't have to scramble, you are definitely more fortunate that those who end up with no position for the next year. If you ranked the places that you would accept a job and you received a position, you are ready to start your residency. No one who goes through the scramble would ever say that the situation is wonderful but it's part of the experience and with that experience comes wisdom.