And then there's the definition Garrison Keillor uses --
"A folk song is one you learned from someone else, and remember the words to, mostly."

Folk songs and tales are to be cherished because they explain ways in which groups of people understood their world and managed their experiences and expectations. What with modern media saturation, we may never have a time again in which a true (i.e., localized) "folk" culture can flourish. But we can still enjoy, and learn from, the existing treasure troves.

BTW, to the list up there, I'd add:
Lullabies
Spirituals
Nonsense songs