Is folk music a simple category to define?

No, not at all. And that, in itself, is kind of the point behind folk music. Unlike rock, or pop, or country, there are no conventions or rules to bind the music. Folk music is an expression of the individual, often narrative in form, never confined by or expected to conform to the social norms of the time. Folk music often has a political or sociological message hidden (or not so hidden) in the lyrics, and is often characterized by simple and/or acoustic instrumentation. But then again, there are artists who don't do any of those things, and they still consider themselves part of the folk music scene.