I'd say plastic of some sorts. I cut some up and put it on my light switch covers around the house. I was trying to be conservative and only cut up little 1/4" squares. While they work fine, I should have cut 1/2" squares.
Oh and BTW, when I got mine (it's been a couple years I think)...they had the instructions right next to the top sheet of the "skin" and some of the black letters transferred to the "skin". I contacted them and they wouldn't replace it. A little reading on my part (somewhere in the archives I believe), led me to believe that it had happened to others as well. Maybe they're shipping differently now?
I thought it was pretty spendy stuff $20 for 2 5"x7" sheets of "skin". Maybe there are better alternatives from our
GlowInc.com (not sure they sell a "skin"/self-adhesive type product though) or another vendor?