The above posters are indeed correct. (for the sake of simplicity, i'll ignore the SF Match and military match)
Applications to residency is handled through ERAS
Then you apply for the match. The ACGME (MD) programs are through the NRMP. The AOA (DO) programs are through the NMS (national matching service). As a DO, you are allowed to apply to both matching service, ACGME only, or AOA only. Each matching service requires you to submit a rank list (
although you may choose to submit no list)
The NMS match takes place in February. The NRMP match takes place in March. By established agreement, if you match via NMS into a DO program, you are automatically withdrawn from the NRMP (MD) match. If you apply for both DO and MD match, and go unmatched in the DO match, you are still in the running for the NRMP (MD) match.
Some websites of interest
AOA Matching Program (NMS)
http://www.natmatch.com/aoairp/index.htm
NRMP Matching (MD)
http://www.nrmp.org/
ERAS (application service)
http://www.aamc.org/students/eras/