NRMP and ERAS are different organizations and systems in the matching process with very different functions, but you may need to use both services to match.
The Electronic Residency Application Service or ERAS is a service provided by the Association of American Medical Colleges (AAMC). ERAS provides a standard application for most residency programs and many fellowship programs that also participate in the Match.
The National Resident Matching Program or NRMP (also known as The Match) is a separate organization and system. After applying to programs and interviewing, the Match provides programs and applicants the ability to submit their training preferences through rank order lists. Those lists are then processed using a Nobel Prize winning matching algorithm to make the best match possible for both applicants and programs.
You must register with both ERAS and NRMP if you would like to use the services of each organization.