From the eight Mitzvot of Pesach (see here) six of them, five prohibitions and one active commandment, are related to Chametz. The Tora forbids us to consume Chametz or anything containing Chametz during Pesach. The Tora strictly forbids also the possession of Chametz, and command us to dispose of our Chametz, right before Pesach begins.
To fully understand and observe the laws and customs of Pesach we must know first what Chametz is.
To be very precise:
1. Chametz is any fermented substance coming exclusively from one of the following five cereal grains: wheat, barley, oats, rye and spelt.
2. Chimutz, the actual fermentation of these grains, only takes place after eighteen minutes from the moment one of these grains or a by-product of these grains comes in contact with water. Some common examples of Chametz are: Bread, pasta, crackers, cookies, candies, noodles, some baby foods, dietary supplements (usually drinks) containing fibers, soup powders, whiskey and beer. Besides whiskey and beer, many other alcoholic beverages contain grain alcohol, which is a Chametz by-product forbidden in Pesach. Years ago Vodka, for example, was made exclusively from potato, but lately, most brands of Vodka use cereal grain alcohol (wheat, barley) which of course renders them absolutely forbidden for Pesach. (See here).
Rice or corn and their derivatives, like corn starch or corn syrup (these last two elements are extensively used in the pharmaceutical and food industry) and any other seeds which are not one of the five mentioned grains, are not considered Chametz, even when they have fermented.
In the coming days we will learn why some Jewish communities--Ashkenazi and Moroccan, for example--do not consume rice, corn and other seeds or seeds products during Pesach, despite the fact that they are not Chametz.