So, I've just been blindly throwing out baking recipes for three years without explaining why they work. For those of you who try a recipe off the internet and can't figure out why you don't get a proper result, a little knowledge about leavening can help you fix the problems in most baked goods. The kinds of leavening agents I use are, in order of complexity: air, steam, baking soda, baking powder, and yeast.
Air: This refers to mechanically beating air into a batter, which then solidifies when the starches or proteins in the product set. It is what makes meringue fluffy and sponge cakes light. It makes whipped cream stiff. When you force millions of tiny air bubbles to stay in suspension, they create volume.
Steam: This is what puffs up cream puffs, Yorkshire pudding, and puff pastry. It is responsible for the flakiness of pie crusts, biscuits, and croissants. It's when you put something moist in your batter or dough (usually water or butter), then bake it at high heat for a short period of time. The moisture creates steam quickly, which stretches out the dough. The dough then gelatinizes. When the steam bakes away, the solid shell of baked product remains. It's like when you make a papier-mâché balloon, then pop the balloon inside once the paper has dried.
Hope this clarifies some recipes and contributes to successful baking.