Unleavened bread contains no yeast, baking soda, baking powder or other leavening agents that allow the dough to rise, while leavened breads are made with rising agents that allow the dough to release gases and expand. Unleavened bread is flat and thin, and it resembles a cracker.
Leavening by air is accomplished with vigorous mixing that allows air bubbles to form in the batter. Fermentation can also be used to leaven batter, by allowing carbon dioxide to release in the dough. Unleavened bread can be made with just water and flour. Matzo is a kosher unleavened bread that is used in traditional Jewish Passover celebrations.