Howard is right, except for one point. The broader meaning of the word 'folk' isn't defined by style, or Ed Sheeran would be seen as a folkie, Jim Moray wouldn't be, and the world would have gone mad. If it's defined by anything it's the venue. 'Folk' in the broader sense is 'everything you hear at folk clubs and folk festivals'. It's a definition by context, like 'rugby songs' or 'Girl Guide songs'.

But that means it's not a definition that tells you anything about the song, the accompaniment or the presentation, except perhaps that it's not likely to be ultra-professional. The accompaniment could be guitar, concertina, bodhran, shruti box or nothing at all; the song could be by Richard Thompson, Cole Porter or Thelonious Monk, or it could beone of your own. 'Folk' in that sense has no boundaries and no content - if you tell someone you sing folk, all they know for certain is that you sing.