NDB stands for Non Directional Beacon. It is a L/MF radio station which transmits a carrier wave with identifier that can be received by an ADF (Automatic Direction Finder) receiver and an indicator in the aircraft to show the direction the station is located. It can be used to determine your own position, as a marker for an ILS procedure, locator for an airway or as backup for the VOR.
If you own an ADF receiver with an extended range you could also receive AM radio stations as a backup or just for your pleasure while flying. As with the VOR page, we discuss operating this device and its technicalities without getting into much detail. Enough to be found elsewhere on that detailed subject.
Eurocontrol published a document containing VOR/DME/ADF radio navigation aids full with tips for pilots.
Eurocontrol Guidance Notes
Excerpt from the document:
The three most common types of radio navigation aid are: VOR indicates the magnetic bearing from the station to the aircraft (the Radial). The original instrument, the Omni-Bearing Selector (OBS) indicates the aircraft's position relative to the radial the pilot has selected, and requires careful interpretation.
ADF (Automatic Direction Finding), indicates relative bearing to a radio beacon, called an NDB (Non-directional Beacon). The original RelativeBearing Indicator (RBI) only shows that relative bearing, and can still be found.
DME (Distance Measuring Equipment) indicates the distance in nautical miles from the beacon to the aircraft. The display is usually digital.
To continue reading see the pdf below: