Chametz is "leaven" — any food that's made of grain and water that have been allowed to ferment and "rise." Bread, cereal, cake, cookies, pizza, pasta, and beer are blatant examples of chametz; but any food that contains grain or grain derivatives can be, and often is, chametz. Practically speaking, any processed food that is not certified "Kosher for Passover" may potentially include chametz ingredients.

Full article: What Is Chametz?

Chametz is the antithesis of matzah, the unleavened bread we eat on Passover to recall the haste in which we left Egypt, and the humble faith by which we merited redemption. Matzah is the symbol of the Exodus, a central component of the Seder rituals, and the heart of the "Festival of Matzot" (as Passover is called in the Torah). And the flip-side of eating matzah is getting rid of chametz — and the egotism and spiritual coarseness it represents.

Full article: What Is Matzah?