NDBs were used in the Maritime Service long before the Aeronautical Service.
Any vertically polarised LF/MF radio transmitter will act as an NDB as the signal in theory, radiates in all directions. The advantage of the NDB for aeronautical use is the cone of silence vertically above the beacon which givies a precise overhead hence their use as airway markers before the VOR. Broadcast stations may also be used but their high power, modulation and variable polarisation can detract from the accuracy.
ADF is "Automatic" DF as opposed to Manual DF that was predominantly used on ships and early aircraft.