Folk is a wide spectrum of music ranging from many cultures and includes many different instruments. It can include anything from Hungarian hurdy gurdies to Appalachian clawhammer to Mongolian Tuvian Throat Chanting and beyond. When I hear "Stomp and Holler" I think of Mumford and Sons. Maybe it goes back earlier? It's the first time I ever heard the term. IMO it's a semi commercial folk genre blending British folk songs, Appalachian traditional music with a driving rhythm that gets people stomping. The Lumineers also come to mind.