Windstopper is a fabric laminate made by W. L. Gore & Associates. It is similar to Gore-Tex, except that it is only windproof and breathable, not waterproof. One of the most common applications is a lamination with Polar fleece, since the lack of wind resistance is one of the principal drawbacks of that fabric.
Windstopper garments are marketed as "softshells" being suited to high-output aerobic activities such as running, cycling or cross-country skiing. Because they are not waterproof, they are more breathable than traditional Gore-Tex clothing. However, because they are based on a solid laminate layer, they remain inherently less breathable than other "softshell" fabrics which provide such as those made by Polartec or Schoeller, which do not use laminates. Of course the drawback to this is those fabrics lose their "windproofness" more quickly than Windstopper clothing.
Like most softshell fabrics, Windstopper products are typically coated with DWR (Durable Water Repellent), which provides them with a modest degree of water resistance. They will wet through in heavy rainfall but can be worn comfortably in light drizzle.