what is chametz...?

Chametz is any food product made from wheat, barley, rye, oats, spelt, or their derivatives, which has leavened (risen). Our sages have determined that flour from any of these five grains that comes in contact with water or moisture will leaven, unless fully baked within eighteen minutes. As we are commanded by the Torah, if a food contains even a trace of chametz, we don’t eat it, we don’t derive benefit from it, and we make sure not to have any of it in our possession for all the days of Passover.
To be certain that a product is kosher for Passover, it must have rabbinical certification. Otherwise it is possible that it contains chametz ingredients, or traces of chametz if it was processed on the same equipment as chametz products. Thus, unless a product is certified Kosher for Passover, we consider it chametz, and make sure not to have it in our possession on Passover.
Note: Matzah used all year round might be pure chametz, and not for Passover use. Only matzahs baked especially for Passover may be used on Passover.
Source