Potato starch flour, also known as "potato starch," is a key ingredient in Scandinavian baking. Since wheat flour was for a long time a luxury item in Scandinavia, difficult to grow and normally thus imported from warmer climes, Scandinavians depended much more heavily upon the hardier potato starches and flours they could produce locally.
Unlike potato flour (which because of its strong potato flavor and heavier weight should never be substituted for potato starch in baked goods), potato starch flour / potato starch has a neutral taste, and is an excellent choice for thickening sauces, gravies, stews, and soups. It also produces superior flour-free sponge cakes, for it absorbs and retains moisture to a far greater degree than wheat flour and produces cakes with a lighter texture.
Potato starch flour / potato starch is gluten-free (unlike some brands of potato flour, which may contain wheat). A corn-free kosher ingredient, it is also typically used by Jewish cooks in place of flour and cornstarch during Passover.
Most American grocery stores and specialty Scandinavian markets carry the Swan brand of potato starch, marketed as "Potato Starch Flour." Ener-G markets it as "Potato Starch Flour" as well. You can also find it labeled as "Potato Starch" from distributors including Bob's Red Mill, Manischewitz, Barry Farm, and Authentic Foods.