Courtesy of Justin Fore at Fore Innovations LLC:
AN: Army/Navy, but many in substitute 37° JIC (omits the controlled root radius and class 3 tolerance) Seal is achieved with a 37° taper and seat. No thread sealing is required. Used primarily for connecting hose ends since the nut swivels around the fitting. (instead of hose having to spin) ORB: O-ring boss. Used to interface female fuel system components to AN connectors, such as fuel rail ends. O-ring accomplishes seal, so no thread sealant is required. NPT: (pipe thread) A tapered thread, often used as a low cost substitute for ORB, also requires use of a thread sealant to seal spiral leak inherent to NPT design. Always use a teflon based liquid thread sealant on fuel system components with this interface...do NOT use teflon tape!