NDBs, or Non-Directional Beacons, are simple radio transmitters that simply transmit a signal in all directions, that of which carries the morse-code identifier of the station and sometimes a pre-recorded weather briefing. The majority of the equipment necessary to navigate via non-directional beacons is carried upon the aircraft. 

This device, called a 'radio compass', operates on the phenomenon that a loop antenna will receive a varying strength in a signal depending on the orientation to the signal - with the greatest strength receiving on the 'side' of the loop, with the least strength when the loop is facing towards the source. 

Using this property of loop antennas, it is possible to fairly accurately determine the source of a radio transmission. Early models of radio compass required the pilot to manually rotate the antenna, and listen to the signal to determine a bearing, however modern systems are capable of automatically tuning, listening, and displaying the bearing upon the compass dial to the pilot. 

NDB symbol, as displayed on a Sectional [map].
https://upload.wikimedia.org/wikipedia/commons/thumb/1/1b/Pictogram_NDB.svg/1024px-Pictogram_NDB.svg.png

An AN/ARN-6 Radio Compass
http://jproc.ca/rrp/rrp3/argus_arn6_id91b.jpg