This article is from the Running FAQ, by Ozzie Gontang with numerous contributions by others.
A teflon based membrane with microscopic holes. Gortex's claim to fame is
that it will let water vapor (from perspiration) through, but not liquid
water (rain). It blocks wind fairly well too. The membrane is delicate, so
it always comes laminated between 2 layers of other material. It does not
breathe enough. There are less expensive alternatives.