A non-directional beacon, or NDB, is a piece of equipment used for navigation - primarily by aircraft. It is 'non-directional' because it sends an identical signal in all directions, in contrast to a VOR station. Thus, the recipient cannot determine their bearing from the beacon; they can only determine the relative bearing to the beacon. In order to navigate using this information, the pilot will also need their own heading. Given the heading of their aircraft and the relative bearing of the NDB, it is possible to determine the absolute bearing to the station.
NDBs are specified and standardized by the ICAO, ensuring that aircraft of all nations can use them. They broadcast on a single frequency in the range of 190 to 1750 kilohertz. At those frequencies, NDB signals travel via ground wave propagation - bouncing from the ground to the ionosphere and back. This permits them to travel 'around the curve' of the Earth's surface, unlike VOR and other navigation systems signals, where the receiver must be within line-of-sight of the transmitter. As a result, they are useful at longer ranges and at lower altitudes.
The NDB signal is received using an instrument called the ADF, or Automatic Direction Finder, inside the aircraft. The signal usually transmits an identification signature in Morse code, so that pilots can confirm that they have tuned into the correct beacon. NDBs are represented on sectional charts and instrument flight charts with their positions marked precisely, allowing pilots to navigate using their signals.
In the United States, nearly all NDBs are operated on frequencies of between 190 and 530 KHz. This is because the commercial AM radio band stretches from 530 KHz to 1700 KHz. However, since the ADF system can receive these frequencies as well, the ADF can be used to listen to music or talk radio in the air, if you so desire. On a more useful note, many AM station antennas are marked on aviation charts and labeled with the station call letters - so AM antennas can also be used as NDBs. NDBs may also be used to transmit relevant information to aircraft such as weather, in the form of an ATIS, AWOS, ASOS or AWIS system. In some cases, NDB transmitters can be used as backup voice transmitters for Air Traffic Control.
NDB installations are sometimes paired with DME stations - Distance Measuring Equipment. There are distinct chart symbols for an NDB vs. an NDB/DME.