The walls are a vertical paneling. This paneling is often referred to as vertical ship-lap or sometimes "nickel board" (because the gap was historically set to the width of a nickel). Because the material is vertical you could also use 4x8 sheets with grooves cut into them to resemble individual pieces of wood. You will need blocking horizontally to nail to. There is no sheetrock behind this paneling...it is nailed directly to the wall studs and horizontal blocking.