An interesting quote from the introduction to The Penguin Book Of English Folk Songs:
"An old Suffolk labourer with a fine folk song repertory and a delicate, rather gnat like voice, once remarked: "I used to be reckoned a good singer before these here TUNES came in". The Tunes he spoke of with such scorn had come in with a vengeance, and it seemed like his kind of songs, once so admired, would be lost under the flood of commercial popular music."
This got me thinking about the nature of folk music, and of English song in particular. These songs were specific to a particular area, or way of life, or mode of being. They were written to celebrate common experience, but in a much more small scale way. This is why a song like John Barleycorn has so many regional various. Raymond Williams (the great thinker of the new left) noted that culture is ordinary, that it is made and remade as people live their lives and seek ways to express that. This seems to me to be particularly pertinent to the idea of folk song.
Popular song, on the other hand, does the opposite. It is a cultural expression that is offered to us wholesale - it is not something that we can hope to participate in ourselves unless we choose it as a career (and I say this as someone who spends a good deal of his life enjoying popular music!)