What is the difference between folk music and acoustic/mellow music?

not everyone artist who does acoustic/mellow music is folk music
(which is what i thought) so what is the difference? what genre do
artists like Get Cape Wear Cape Fly, and Benjamin Francis Leftwich
...come under ?

Get Cape. Wear Cape. Fly

http://www.youtube.com/watch?v=b7kmcEF8Hgo

Benjamin Franics Leftwich

http://www.youtube.com/watch?v=sin9PRz583E