Very thin nylon type bag. Hope it’s water resistant.
I use these for my weekend bag that I take to my boyfriend's and they're super sturdy. I use one of them for dirty laundry and then wash it with the clothes when I get home and it's holding up! The only downside is that the stitching seems to be wearing out on both of them near where the zipper is and sometimes the zipper gets caught in the strings. Not much to complain about for a dollar, though!