The Hurdy Gurdy is an instrument that has been around for almost a thousand years. It has been in continuous use all of this time so throughout Europe it is known by a large number of names. France: Vielle a Rue Germany: Drehleier Italy: Ghironda Spain: Viola de rueda, Zanfoña And there are many others across Europe and the Eastern Block Countries.
It was first invented in a Monastry in what was to become North Germany, to teach Monks how to sing and show them a bit of music theory (string lengths and proportions). The earlier instrument was played by two players and called an Organistrum. One player turned a handle which rotated a wooden wheel which bowed or rubbed the strings from underneath, while the other player moved wooden tangents which touched the strings to change the length and there fore the sound or pitch.
The next version was a one person instrument called a Symphony (after the Greek for many sounds). In this shoe box sized instrument it was found that you could add extra strings to the wheel so that you had drone notes. These are continuous notes that create a harmony when the melody or Chanter strings are added (similar to the sound of a bagpipe)
The complete and modern day version has an extra string that controls a bridge (piece of wood) shaped that it is hinged at the back and loose at the front which is in the form of a little arch. A string runs across the top and when this string is tensioned properly against the bridge it forces the little bridge to jump up and down with any extra jerk of the wheel. This creates a rythmic buzz.
Here is a (You Tube link)