When I first started baking, I noticed that some of my recipes called for cake flour, while others required all-purpose flour. Also, I had never really noticed before that the baking aisle at the grocery store is full of flour choices in addition to cake flour and all-purpose flour—like bread flour, pastry flour, and self-rising flour. Silly me, I thought flour was flour!
I decided to research the differences between the flours. I learned that flour is usually made from hard wheat, soft wheat, or a blending of the two. The harder the wheat, the higher the gluten content. Gluten is a protein that causes yeast to have its elasticity and to rise.
All-purpose flour is a mixture of hard wheat (high gluten) and soft wheat (low gluten). It has between 10-12% protein content. This is why all-purpose flour is so versatile, and you can use it to make things such as cookies, biscuits, and some cakes. Cake flour is a bit of a different story. It is made from soft wheat (low gluten), and it has about a 6% protein content. This is why cakes made with cake flour have a bit of a different texture—the crumb is more tender.
Although the flours are quite different, you can substitute all-purpose flour for cake flour (or vice versa) in your recipe, if you make a few adjustments. If you need cake flour but just have all-purpose flour on hand, just take one cup of flour, remove two tablespoons of flour from the cup, and then add two tablespoons of corn starch. To substitute cake flour for all-purpose flour, use two cups plus two tablespoons for each cup of all-purpose flour.
While these substitutions can be made, if you can go to the grocery store to get the flour that your recipe calls for, that is your best bet. I have found that baking is a lot like science class, and cakes can be fickle, sometimes. So, for the best results, follow the recipe exactly, including the list of ingredients.