The term Folk Music has come to encompass a wide variety of styles, traditional and contemporary, based mainly on the musical roots and traditions of Europe. It includes:
Regional and ethnic traditional musics of the continent, performed by people for and within their own community, passed on and evolving through live performance.
Recorded collections of the aural tradition.
Composed songs and music which have become so well known that they can be said to have "entered the tradition," and are evolving through repeated performance.
Interpretations of the tradition, using different vocal styles and instrumentation.
Maintainers and interpreters of European traditional musics in diaspora communities of North America and Australasia.
Since the 1960s folk revival, particularly in North America, the UK, and Australasia, it has also come to include music of singer-songwriters, who sing their own (usually personal and introspective) material, with no direct connection to the original folk tradition other than style of performance.
http://www.nigelgatherer.com/ Resources for people interested in traditional music; includes instrument tutorials, tune collections, session listings, profiles of Scottish performers, and a bibliography.