Before we discuss the differences between yeast and quick breads, we must first understand the process of leavening. A leavening agent is a substance used in batter or dough, which enables the finished product to expand and hold its shape. The most common leaveners, such as baking powder, baking soda and yeast, create bubbles, which are caused by a biological or chemical reaction, and therefore allows the finished product to increase in volume (source). Possibly the main difference between quick and yeast breads is the type of leavening agent that is used.
Yeast breads utilize yeast (typically active dry yeast for us home bakers), and is leavened due to the fermentation process. When the yeast comes in contact with a warm liquid and allowed to rest, it biologically changes the chemistry of the dough by reproducing and consuming carbohydrates (including sugar) in the dough, therefore producing carbon dioxide. This is what allows the bread to rise. Before baking, yeast breads also require kneading and are allowed to rise (also known as proofing), usually 1-2 times. Some yeast breads include white and whole wheat bread.
Unlike yeast breads, quick breads don’t use yeast. Instead, they use agents such as baking powder and baking soda. These agents allow quick breads to leaven more rapidly than yeast breads, hence the name “quick bread”. These breads utilize a chemical approach, which causes the leavening agents to react when they come in contact with heat and liquid. Unlike yeast breads, quick breads do not require kneading or rising, but instead rise when baked. Some quick breads include banana bread, biscuits, brownies, cakes, cookies, pancakes and scones.