NDBs, or non-directional beacons, are low and medium frequency aids, which operate on a band between 190 and 535 Khz. They are just like AM radio... In fact, some NDBs double as AM radio stations, and you can turn up the volume on an NDB and listen to the the radio in flight. I wouldn't reccomend this, because you might miss some important flight communication, and it's AM, so their isn't usually anything good on anyway.
So, an ADF (Automatic Direction Finder) Is the instrument that is used to navigate from NDBs. It consists of simply an arrow pointer inside a compass rose. The arrow simply points to the station. That's it. ADFs usually have non-slaved compass cards (the card is the compase rose) however, meaning that there is no connection between it and the compass or directional gyro) So you'd have to adjust that to your actual heading, and then when you make a heading change to get the actual magnetic bearing to the station.
ADFs are subject to a few errors. They are vulnerable to precipitation, lightning, static, etc., and at night they are subject to the same interference from distant stations that normal Am radio is. It's interesting: When lightning strikes, the ADF needle will momentarily point to where the strike occured. ADFs have no flag to warn of unreliable navigation information like VORs do. In this case, the AIM reccomends turning up the volume and continuously monitoring the NDBs identification, because almost all errors also affect the identification signal.
I hope this helps. Let me know if you need any clarification.
Jack @ AUS