Cake flour is made from soft winter wheat. It has 8 grams of protein per cup, compared to 10 to 12 grams in all-purpose flour. Less protein means a more tender cake. It also means more starch is available to swell and stabilize the cake. Cake flour is more finely milled which helps batters come together quickly which means less gluten formation which means tenderness. Cake flour is chlorinated and therefore slightly acidic, this helps the proteins in the flour to set faster and contributes to a finer texture. Thanks to Alton Brown - Good Eats for tip.