This is as good a simplified definition of Folk Music as any
Jim Carroll

FROM THE OXFORD ENGLISH REFERENCE DICTIONARY.
FOLK MUSIC n. instrumental or vocal music of traditional origin transmitted orally from generation to generation, whose authorship is often unknown. Folk music tends to have a relatively simple structure and melody, and to use portable instruments such as guitar, violin, harmonica, accordion, and bagpipes. Folk music is often monophonic, consisting of simple unaccompanied tunes although vocal polyphony is common in southern and eastern Europe. While some regions of Europe (e.g. Bulgaria, Romania, the Basque Country, Macedonia, etc.) there has been an uninterrupted there has been a living tradition of folk music, concern began to be felt in the late 19th in Britain and elsewhere that the folk tradition would be lost. Pioneering collectors and revivers of folk music include Cecil Sharp and Percy Grainger in Britain, Dvorák in Bohemia and Moravia and Kodály in Transylvania. During the Depression years in in the U.S. Woodie Guthrie revived interest in the form with his political protest songs: he had a strong influence on later figures such as Bob Dylan, who later performed traditional folk material and also wrote new songs in a folk style. In Britain in the 1960s and 1970s groups like Fairport Convention and Steeleye Span explored and expanded the form; electronic and other less traditional instruments began to be used, giving rise to a style known as folk rock.
Recently much attention has been given to folk music from other cultures (see World Music)