In your fourth year of medical school, you must decide which field of medicine (specialty) to practice. Each medical field requires a residency (a training period — sort of an apprenticeship) of three or more years.
Information about residency programs can be obtained online from the American Medical Association's FREIDA (Fellowship and Residency Electronic Interactive Database) website. After deciding which specialty to enter, you must decide which residency program you like, then go visit and interview there. This is where the "Match" comes in.
The Match is run by the National Resident Matching Program (a non-profit corporation). They conduct a sort of computer matching game. It works in the following way. After everyone has completed their interviews, the graduating medical student submits their list of residency programs in order of preference. At the same time, the residency programs rank the students that they prefer. The NRMP enters the info into a computer to match students and residency programs with their highest possible preference. Every graduating medical student finds out the results on the same day in mid-March during "Match Week".
You are committed to accept the position you have matched into. If you did not match into a residency you are informed a few days prior to match day. A list of unfilled positions is provided to the unmatched medical students so that they can participate in the Supplemental Offer and Acceptance Program (SOAP), also known as the "Scramble," which helps to place unmatched medical students into unfilled positions.
In the US, physicians complete a residency in one of numerous specialties. A residency takes a minimum of three years. Sub-specialty training (called fellowships) take additional years to complete.