In the modern terminology used among the youth of this mighty nation, someone playing an acoustic guitar is likely doing Folk. If a violin or mandolin or banjo is also heard, it is definitely Folk.

Among the past winners of the Grammy for Folk Album of the Year...Roger McGuinn for an album that featured Roger, Judy Collins and Jean Richey performing traditional music...and Steve Earle for an album of self-penned songs featuring amplified instruments and thundering drums. But I think Steve had a mandolin in there somewhere.