Those are just pine boards and I think they're 1x12's. I believe they are just butted together, though some people like to use a ship lap. The boards were nailed directly to the studs and in this type of application (butt boards), it's useful to use a black felt or poly backing so you don't see through the joints.

I just got back from visiting one of our houses under construction that used chip board (painted black) as a substrate for boards on the wall....