What exactly is folk music?

I've been interested in Bon Iver recently, and everytime I research the band, they're called a 'folk band,' but I don't have a good grasp of what that really means. What separates folk music from other genres of music? The internet hasn't really given me any good answers to this question, so I thought there may be someone on here who can give me a better answer. I'm not looking for an essay to read-I just want an explanation of what makes folk music unique to other genres.

music