Like the Instagram photo effects??
Well, my answer to that now is much different than the answer I would have given you 2 years ago. 2 years ago I would have said that it didn't matter in the least and that any time you see "cake flour" called for in a recipe it means that the recipe author must be a completely unrelateable person who thinks you also have milk in your fridge that came from your grass fed cow Bessie in your back yard. Hmph. Hoity-toity.
As I came to find out, though, there is a difference after all [sheepishly shrugs]. All-purpose flour is a blend of two wheats: low gluten soft wheat, and high gluten hard wheat. Cake flour is all low gluten soft wheat with a very finely milled texture.
Also related and important to note is the percent of protein in all-purpose flour and cake flour. Regular all-purpose flour has anywhere from 10-12% protein while cake flour only has around 6%. *Some southern brands of all purpose flour (such as White Lily) have around 8% protein.*
Fun extra credit tid-bit: Bread flour has an even higher protein
content, around 12-14% and
is used to give bread it's chewy texture.
Okay, that's great and all, but what the heck does it mean? In layman's terms this means that when you use cake flour you end up with a much lighter, more tender crumb. The lower the gluten/protein is in a flour the more tender your end product will be.
So what do you do when you suddenly decide to whip up a tasty treat that calls for cake flour but you're in your jammies (even though it's 3 pm) and you don't want to get dressed and run to Publix? *disclaimer: I only cite this scenario because it's basically the story of my life.*
The good news is that you can substitute all-purpose flour for cake flour by just slightly altering the amount you use. Simply remove 2 tablespoons of flour from every one cup of regular all-purpose flour. And tada! That's IT.
PS If you're really a science nerd and want the nitty gritty on all this you can check out this awesome article from Joy of Baking here.