Flour
Flour provides the structure for the product. The gluten, or protein, in flour, combines to form a web that traps air bubbles and sets. Starch in flour sets as it heats to add to and support the structure. In cakes, cookies, and quick breads, we want little gluten formation, which makes products tough. Fats and sugars help prevent gluten formation. In most baked goods, all-purpose flour is a good choice; it has less gluten than bread flour.
Fat
Fat coats gluten molecules so they can't combine as easily, contributing to the finished product's tenderness. In many cakes, fat also contributes to the fluffiness of the final product. When sugar is creamed with fat, small pockets of air form from the sharp edges of the crystals interacting with the fat. These pockets form a finer grain in the finished product. Fats also carry flavors and add to a tender mouth-feel.
Sugar
Sugar adds sweetness, as well as contributing to the product's browning. Sugar tenderizes a cake by preventing the gluten from forming. Sugar also holds moisture in the finished product. Sugar crystals cutting into solid fats like butter help form the structure of the product by making small holes which are filled with CO2 when the leavening agents react.
Eggs
Eggs are a leavening agent and the yolks add fat for a tender and light texture. The yolks also act as an emulsifier for a smooth and even texture in the finished product. And the proteins contribute to the structure of the baked good.
Liquid
Liquid helps carry flavorings throughout the product, forms gluten bonds, and reacts with the starch in the protein for a strong but light structure. Liquids also act as steam during baking, acting as a leavening agent and contributing to the tenderness of the product.
Salt
Salt strengthens gluten and adds flavor. Salt enhances flavors. In yeast breads, salt helps moderate the effect of the yeast so the bread doesn't rise too quickly.
Leavening Agents
Baking soda and baking powder form CO2, that is held by fat pockets, gluten and starch, which makes the baked product rise. Baking soda and powder are not interchangeable; be sure that you have the product the recipe calls for. To much leavening agent will make the bubbles too big, then they combine and burst, leading to a flat cake or bread. Too little leavening agent will result in a heavy product, with soggy or damp layers.
