NONWOVEN POLYPROPYLENE, a form of plastic—a flexible resin polymer.
It looks and feels flimsy, I picked it up in the store and said "naaa," but a week later re-looked it and decided to try it. Glad I did, as the held up well. I don't recommend picking them up by the handles and swinging them around with 30Lbs of wet soil in them, but same goes for SmartPots or GrowBags.
We have something similar called an eco bag. Thanks for the tip!