This article is from the Bagpipe FAQ, by Denise McNickle email@example.com with numerous contributions by others.
The type of bag you're referring to is commonly called a Canmore bag.
Canmore bags are made of a synthetic fabric called Gore-Tex. Some people swear
by them: they're lighter and easy to maintain (never need seasoning). Other
people swear at them, saying that they are too light and that the mechanism
used to hold the stocks (o-rings) is too unreliable.