They're polyester and are semi-translucent. We've used them for the curtain itself in all of our showers.
They're mildew resistant and can be thrown in the washer. They are not that stiff vinyl that you find in many liners but a fabric-y, soft polyester. Finished with silver grommets and weighted bottom hem, these do the job simply and elegantly. Available in 6' and 7' lengths for $12 to $30.