Leavening of anything can be done in three different ways. The first would be to steam leaven something, as in a souffle, or popover. Usually whipping egg whites, or using a very light batter to cause the expansion of water into steam giving the item a lift during the baking process. The second way is chemical leavening, as in biscuits, quick breads. Involving baking powder or baking soda, these items sometimes will require a quick rest (allowing the flour to bloom- or hydrate) but usually will go directly into the oven once assembled. The term "double action" in baking powder refers to the fact that it will release gasses once when assembled, and then again when it reaches around 150 degrees. This is a bonus for a cook especially in the world of pancakes, and waffles, resulting in a more "fluffy" final product. The third way to leaven is organic leavening which is just a fancy term for yeast.
Yeast has been a staple in humanity since we first decided to ferment grapes or make beer, or make any sort of bread. Originally harvested from decaying fruits, or simply allowing a mixture of flour and liquid to attract the natural yeasts that are in the air all around us. While a really great idea, giving yeast a undeniable flavor that differs from the flavor you could get 30 miles down the road, it isn't really a possibility in today's world. The actually rise of the dough is the yeast multiplying and dying, that process creates carbon dioxide, and ethanol which not only develop flavor, but also help to develop the gluten structure of the dough. Yeast is alive in your bread dough all through the mixing and rising process even into the baking process. The oven will put the yeast into hyper drive during the first part of the baking process causing what is called "bakers spring" the quick growth of a dough in the first few minutes of the baking process due to the ideal conditions. The different types of yeast most bakers and home cooks use can be confusing and I thought I would try to clarify.
Brick/Fresh/Cake Yeast- A compressed form of yeast that is still alive and kicking. It dissolves into a paste when mixed with water. While its flavor is amazing, and I think it is more "yeasty" than the other yeast options it is very temperamental, especially if you are used to working with the more refined products. I have lost a whole restaurant size batch of wheat loafs to it. Not because we didn't know what we were doing, but more because it reacted differently than the other yeast we usually use. It cant be beat for flavor, but beware. Even the most aggressive home/restaurant bakers I know don't prefer it. If you are dead set on giving it a shot (you should at least a few times) then the cakes will have to be acquired from a specialty baker store of some sort, or of course online. Much of it's varied reaction can be due to its age, as it is alive and working. It must be stored in the fridge, but not in a airtight setting, and should be used within a week or two of its production date.
Active Yeast- this term really kind of cracks me up as no one would want inactive yeast. Active yeast is what most people use. Often coming in small packets (their measurement is 1TB). This yeast has been dried and then given a coating to make it go dormant. Because of that it needs to be "awaken" and that is best done in a quick soak in warm water. When proofing yeast in water you want only to include the water and yeast. All yeast reacts poorly to salt, and even large amounts of sugar. The nice thing about the process is that it only takes a few minutes and I think it actually speeds up the rise of the dough once assembled. You have given it an ideal place to wake and get to the business of multiplying, therefore when mixed with the dough it is already alert and in maximum production mode. Also best stored in the fridge, and in its individual packet form it should be good for more than a year, although if you don't use more than a few packets of yeast a year you should be baking more bread.
Instant Yeast/quick rise Yeast- this yeast has been quick dried, but does not have the coating that active yeast does. It can do directly into a mixing bowl and will activate "instantly" with the addition of water. I can find it in almost any grocery store now, and it is my best choice for home usage. It is so predictable, flavorful, cost effective, and easy to use. It needs to be stored in the fridge, but a friend of mine taught me to store it in the freezer, and I have never gone back.
A big misnomer about yeast is that it needs to be measured. A tsp of yeast will do the same leavening as a TB, just much slower. But it is important to remember that while yeast multiply, burp, fart, and die inside your bread dough they develop flavor. Slower is always better. Letting a bread dough rise once, and then putting it in the fridge overnight to rise again tomorrow will always develop more flavor than a rise, punch, rise, bake. That being said never let the fact that you don't have a dough in the fridge stop you from making a loaf of bread for dinner tonight. Bread making at home is one of the most gratifying things I experience in my kitchen at home. People don't expect it, people don't do it anymore, and once you get the hang of it, it becomes pretty routine. The smells involved, and the example of all of that knowledge and work in a finished form on a table for my family and guests shows an over the top effort to make a meal enjoyable, and healthy for them. At the end of the day that is the whole reason we cook.