Leavening agents main function, as their name states, is to leaven baked goods. Whether it's a crusty loaf of bread or a tender angel food cake, leavening agents can also tenderize and provide a finer crumb structure. Also, if using baking soda, this leavening agents adds flavor (think baking soda biscuits).
Physical leavening agent: Steam
Some baked goods count on a big burst of steam to get the rise they need. For example, pate a choux, used to make profiteroles, are baked at a high temperature for the first 10 minutes in the oven, making them "puff" due to the quick steam production.
Biological leavening agent: Yeast fermentation
Although the first breads were flat, like tortillas and naan bread, wild yeast was captured and later used in "bread mash" to ferment bread doughs. This method is still used today, as some bakers would rather use the wild and local yeast around them then store-bought yeast. Known as a sourdough starter now, this bread mash has become the "mother" to bakers around the world. By allowing wild yeast to ferment flour and water, you obtain a mixture that can now be the base of all your breads. Some bread bakers are even known to preserve this "mother" by freezing or drying parts of it for re-activation decades later!
What is fermentation? Fermentation is the process in which yeast cells break down sugar for energy. To aid the fermentation of your yeast, and, therefore, your dough, you need to consider dough temperature (very hot or very cold temperatures will kill your yeast), amount of salt (salt is a dough retarder, meaning if there is too much salt, the dough will take longer to ferment), amount of sugar (sugar is what yeast feeds off) and amount of yeast (too little yeast and your dough ferments slowly).
Types of yeast include compressed yeast, active dry yeast and instant yeast.
Try a recipe using yeast as leavening agent: Yeast rolls >>
Chemical leavening agents: Baking soda and baking powder
Baking soda, or bicarbonate of soda, decomposes to form carbon dioxide that produces leavening. But to produce enough leavening, you'd need a lot of baking soda. Thus, this is usually combined with an acid such as buttermilk, yogurt, brown sugar or vinegar to balance out the flavors. Too much baking soda leaves a strong chemical flavor in baked foods.
Baking powders are all made up of the same ingredients: baking soda, an acid and a dried starch or filler. Typically used in leavening of cakes, it results in a tender crust.
Make your own baking powder! Mix 1 part cornstarch, 1 part baking soda, and 2 parts cream of tartar. For example, 1 tablespoon cornstarch, 1 tablespoon baking soda, and 2 tablespoons cream of tartar. Store in an airtight container for longer shelf life.
More baking tips
Baking 101: Choosing the right flour
Baking 101: Gelling and thickening agents
Baking 101: Sugar and other sweeteners