[jcrawfordor]

To somewhat oversimplify, the regional accreditation bodies (about a half dozen of them) are older and better established than the national accreditation bodies. They generally have 100+ year histories, accredit all of the major universities within their jurisdiction, and in general have acquired a great deal of respect throughout US academia.

National accreditation boards in general, and especially those that are not field-specific, are seen as being less rigorous, faster to accredit, and more profit- or prestige-motivated than the regional accreditation boards. Most are relatively new and some of the most prominent (e.g. ACICS) have been involved in specific controversy over the quality of their work. Because the regional accrediting boards are older and better respected, the first question about a university that advertises a national accreditation tends to be "why aren't they regionally accredited?". It is both symptom and cause of this difference that most for-profit and otherwise "questionable" institutions are nationally accredited and not regionally accredited.

While some national accreditation bodies are considered rigorous and respectable (e.g. ABET), they tend to be field-specific (in the case of ABET, engineering) and often require regional accreditation as a prerequisite to accredit a university, as they consider regional accreditation to be the indication that the broader learning institution is able to provide a quality program. Usually specific departments or colleges of a university will go to these types of accreditation boards to add a credential to specific programs (e.g. "ABET accredited computer science program") above and beyond the university already having a regional accreditation.