Quite often when reading threads I see people writing that this or that performer is not really playing "folk" music. So what I'm asking is - What IS folk music?
Is it just traditional stuff or just stuff played with certain instruments or is there a more technical definition to do with structures or rhythms or subject matter?

To paraphrase the late lamented John Peel (bless his soul):
Never quite understood the definition of folk music or world music - isn't all music played by folk for folk in the world?

I'd be interested to hear what people think...