Leavened bread is bread that contains a small amount of yeast or another leavening agent, which causes the bread to rise and become lighter and airier. On the other hand, unleavened bread is any of a wide variety of breads that are prepared without using rising agents such as yeast. Unleavened breads are generally flat breads, and they are denser and harder than leavened breads. Some examples of unleavened breads include matzo, lavash, and tortillas.

The difference between leavened and unleavened bread is found in the word "leaven." Leaven is what is placed in bread dough to make the bread rise. Yeast is a common leavening agent, but there are many other ingredients that can be used, such as baking powder or baking soda. Leavened breads may also contain other ingredients such as milk or fat, which adds to the richness of the bread.

Unleavened bread has symbolic importance in Judaism and Christianity. Jews and Christians consume unleavened breads such as matzo during Passover and Eucharist, respectively, as commanded in Exodus 12:18. The newly emancipated Israelites had to leave Egypt in such a hurry that they could not spare time for their breads to rise; as a reminder of that, unleavened bread is used in ceremonies. Leavened bread is forbidden during Passover because God gave Moses instructions to avoid yeast during the first Passover in Egypt and remove all yeast while obeying these instructions as a lasting ordinance for the Israelites.

In summary, leavened bread is bread that contains a leavening agent, while unleavened bread is bread that is prepared without using rising agents such as yeast. Unleavened bread has symbolic importance in Judaism and Christianity, and it is consumed during Passover and Eucharist, respectively.