Baking almost always involves aerating the flour mixture in one way or another. The yeast in many bread recipes puts air into the mixture, but in cakes, pies and other sweet baked goods in which yeast is not used, just the mixing of the ingredients themselves renders the dough fluffy. Additional techniques and additions can ensure you get enough air into your flour mixture.
The most common and easiest way to introduce air into a flour mixture is by sifting the flour. You can easily substitute a fine-meshed metal colander for a sifter, shaking it lightly from side to side to sift the flour. Once the flour is sifted, measure it without tapping the measuring cup and allowing the flour to settle down again.
You can also trap air in the flour mixture by folding the batter over and over again. Place all of your heavy ingredients in the bottom of the bowl and put lighter ingredients on top. Cut straight down through the mixture to the bottom and up again with a rubber spatula. Rotate the bowl as you do this. After a while, the ingredients will be mixed and airy, all in the same process. After this is done, finish with the rest of your preparations and do not let the mixture linger too long before placing it in the oven.
Egg whites will always incorporate air into a flour mixture. Sometimes they are beaten to a foam before they are added, but even just stirring an egg white with a fork will aerate your mixture. In fact, with the exception of some breads, you will not find many baking recipes that do not call for eggs.
Baking powder helps to create extra volume that ensures the fluffiness everyone craves in a cake. This ingredient contains both an acid and an inert filler. The acidity produces carbon dioxide, and the filler absorbs the moisture you will find in the kitchen air that would otherwise render the baking powder less effective. Most cake mixtures require baking powder.
References and ResourcesKitchen Savvy: Sifting Flour
Baking 911: Fold
"Understanding Food: Principles and Preparation", Amy Christine Brown; 2007