NRTLs test, evaluate and field evaluate to Standards. The Standards are developed through a process that includes different entities.
So, ETL, CSA, TUV, and any other recognized Testing Laboratories (I believe there are approximately 18) follow those Standards as we do the NEC and other codes/Standards.
OSHA is the governmental agency that oversees/provides recognition of the NRTLs. Remember all NRTLs are not recognized by OSHA to test, evaluate, or list
all products, some have a small portion, and others have a very large portion.
UL is the most recognized, as they have the largest presence in that industry.
As a matter of fact, there will be another NRTL added to the list by November of this year.
Here is OSHA'a link to NRTLs
http://www.osha.gov/dts/otpca/nrtl/