It's music that has been handed down through the generations and is accessible, not so complicated so folks can play and learn it quickly.

It is not an academic excercise in exclusivity.

It generally isn't complicated like jazz, classical or swing music.

It lives because of a natural selection. (You could say it was a "meme")

There are different kinds of folk music because there are different cultures.

It isn't rocket science. Generally, you can recognize it when you hear it.

The more complicated it becomes, the less folk it is.

It generally has lyrics that tell a story. (Narrative ballads etc.)

Now the problem will be when you ask the next question. What is a folk singer?
That's the real can of worms, right there.

Frank