Don't forget Public Domain, used often when an author is known, no copyright was ever sought for the work and the author is deceased.

I use Folk and Traditional synonomously because that is the academic usage. Using Folk-styled instead of Folk would allow people all the wiggle room they need to call "Blowing in the Wind," for example, a folk song.

To continue with Frank Hamilton has said above, a real folk song comes from the community. Anyone in the community can say, "Yes. That's part of the life around here." If the song goes, for example, from Scotland to Vermont, it changes enroute and becomes at least as much about Vermont as Scotland.

I struggle with the expresssion Contemporary Folk Song (which I hear a lot nowadays) because

a. contemporary folk songs are rarely the expression of what we can realistically call a Community. They are most often written by people outside of the Community the song is supposed to be from.

b. sing a contemporary folk song and someone will tell you you have the words wrong.

c. most of the time, they don't sound like real folk songs, they sound like pop songs.

All the best,
Dan Milner