What folk music means can be trickier to pin down in the United States as it's such a big place and its history is made of immigration from a big lump of the different populations of the planet. A good start to understand what folk music meant and still means in America is Harry Smith's anthologies as they, in part, embrace different strains of music that entered it with mass-immigration. It's still not complete but Smith has few equals in his diligent study of indigenous music.
It's much easier to define folk music in any European countries or Asia, India, China etc as they either haven't got a history of huge fluxes of migrants flocking in or they have it on a minuscule scale compared to the States.
Then you have the modern approach, still something to be explored in a more detailed way.
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