It feels like paper but really isn't. What I have is called Poly Stable Stuff. description:
It comes by the yard or prepackaged in fifty 8.5" x 11" sheets ready to go for use in a copy machine, ink jet printer or laser printer. You can leave it on the underside of your work. If you wet or wash your finished project, Stable Stuff® Poly becomes a fine, soft layer of unnoticeable polyester fibers inside your project.