The major difference here is that mass culture (or popular culture) is the culture of the great majority of the people while folk culture is a more traditional culture. In some places, this could be the same thing. However, in countries like the United States, they are very different.
Our mass culture today is a culture of television, movies, the internet, and spectator sports. It is a culture where women wear pants and many young men have earrings. This is the way things are in our culture today, but it is by no means traditional.
Folk culture is about how we used to do things. It is acoustic music played among friends. It is homemade things like quilts and perhaps furniture. It is often different in different places because the old ways tended to be very different before the mass media made everywhere much more similar. Therefore, folk culture in Hawai'i might include plate lunch and playing ukuleles and dancing hula while folk culture in Texas might include barbecue and country music played on guitars and banjos.
Mass culture is what "everyone" is doing now. Folk culture is what people in specific places used to do in the old days.