The first difference is related to the type of wheat used to produce them.
Cake flour is made from soft wheat, while all-purpose flour is a blend of soft and hard wheat.
The second difference between cake and all-purpose flour is the amount of the starchy portion of the wheat kernel or endosperm that is used to make the flour. The third difference is the extent to which the flour particles are ground.
Should you run out of cake flour and want to bake a cake, you can substitute all-purpose flour, using 2 tablespoons less per cup.
http://www.aces.edu/dept/extcomm/newspaper/dec3c01.html
cake flour has a lower gluten content. Gluten is the proteins found in wheat. When you make bread, you want a higher gluten content so that it is stronger. The gluten bonds when it is wet and gets worked (kneaded, stirred). If you use all-purpose flour, it has a gluten content that is half-way between cake flour and bread flour so that it can really be used for all purposes in a pinch.
If you use cake flour, you will have a lighter, fluffier cake than if you use AP flour.
I hope I explained that well. I just got up 
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