Cake flour is finer than all-purpose flour; it is made from soft wheat, and is lower in protein. The result: Cake flour makes a more tender cake. To substitute regular (heavier) flour for cake flour, use 3/4 cup plus 2 tablespoons of all-purpose flour for every 1 cup of cake flour called for in the recipe.
And next time you are at the grocery store, grab a box of cake flour to keep in your pantry. It is usually sold in a 2-pound box and has a "Use By" date.