Ok, what is the difference between cake flour and all purpose flour? If the recipe calls for cake flour is it necessary to use that kind?

http://www.wisegeek.com/what-is-cake-flour.htm

it is recommended to use the flour the recipe call for. cake flour tends to produce a lighter cake whereas ap flour produces a heavy denser cake. ap flour has more protein and produces more gluten than cake flour.

There are substitutions available for removing some of the AP flour and using constarch in place of some of the flour.