Folk music is any music made by ordinary folk, even if it's not folk music - whereas folk music is all the music that was made by ordinary folk, even if it's not folk music now.

Folk music, on the other hand, means anything that sounds a bit like James Taylor, June Tabor or Pentangle - whereas folk music means anything that sounds a bit like artists who sound a bit like James Taylor, June Tabor or Pentangle only different.