Cap'n,
If somebody new comes asks about Indian food you don't introduce them to the whole menu - from biriani to phal, but you choose something that will ring bells with them - and not put them off.
With folk music, surely you give them something that they might be able to relate to.
Different cultures have different musics with different origins,. disciplines, styles and objectives.
Sure, you can point them to Ninon Leader's 'Hungarian Classical Ballads and their Folklore' or Jan Ling's excellent 'European Folk Music', or Willard Trask's 'The Unwritten Song', or the works of John Blacking or Ruth Finnegan - and a whole host of other important contributions to our understanding of folk music.
It is my opinion that you start your definition as near to home as possible - would you feel competent enough to explain the music of the Russian Steppes, or the ritual dances of the Cherokee Indians, or Mongolian throat singing - I wouldn't!
There was an excellent series of 13 programmes in the 70s by Bert Lloyd - 'Songs of the People', which covered the whole international gamut of world music, from Pygmies imitating the sound of bees in order to gather honey, to the magnificent complexity of Joe Heaney's singing. Anybody who wishes to take their understanding of folk music further would certainly do well to seek those out (I know there are copies floating around). Or Lloyd's 'Folk Music Virtuoso' -well worth getting hold of.
But I repeat, in my opinion, to anybody asking the above question I would have no hesitation in pointing them to 'Folk Song in England' for a down-to-earth, extremely readable and entertaining answer, and to 'Penguin' for examples of the genre.
Jim Carroll