Actually, it was originally called FOLC Music...

"For Ordinary Low Commoners" (the Margaret Thatcher definition...) :-)

I'd say seriously, however, that folk music in a general sense is music that is created without a specific commercial objective in mind, but rather simply for its own sake. As such it is generally ignored by the mainstream commercial media, except in the case of artists who are so remarkable that they attain broad public notice DESPITE the fact that they play folk music.

Folk music can be in almost any recognizable style, from trad to country to classical to new age to ethnic to celtic to aboriginal to whatever you please. And it can be about any subject whatsoever. It usually includes lyrics, but not always. The lyrics are usually a bit more important to the song than in many other styles of music. Folk audiences tend to listen a bit more closely to the lyrics, hopefully....

It is written and sung for its own sake, because people like it for its own sake.