Q: Thats what hered. WHAT HAS LEAVEN OR YEAST IN IT, i know Bread. but what else?
A: Any grain (wheat, oats, barley) exposed to moisture for more than 18 minutes before full baking becomes "leaven," "chometz." This is counted from the time the grain is harvested, so even regular flour is considered chometz today, on the assumption that there was rain or similar moisture which may have begun a leavening process unnoticed. Thus any product which has any derivative of grains is considered leavened, unless the product is made from grain which has been properly supervised and watched. For this reason, not only are typical grain foods like breads, crackers, cookies, cereals, waffles, and so on, included, but also common things like ketchup which have ingredients derived from grains. There is a second category of foods referred to as "kitniyot" which are grain-like either in their growth or their use. These include beans, peas, peanuts, corn, and even mustard which grows in a similar fashion. With the extensive use of corn by-products in prepared foods, this has very far-reaching consequences. Kitniyot is only avoided by Ashkenazi Jews. While we generally can expect that many foods would not have any of these ingredients, due to the strict nature of the holiday, many observant Jews choose to be particularly strict about this, and will not purchase products which do not have proof they are indeed fit for the holiday. For this reason, for Passover use many people will only buy even goods like apple or orange juice which do have labelling showing that they have been rabbinically supervised. A quick list of things which to Ashkenazi Jews would never be permitted in their usual forms (though fake versions allowed for the holidays are often available) would include bread, crackers, cakes and cookies, waffles, pancakes, breakfast cereals of any form, rice, rice cakes, barley, pilafs, corn, tortillas, noodles, most sodas (it's the corn syrup), most sweetened goods (corn syrup again), flavored coffees (the flavoring agents often include grain derivatives), soy sauce, cous cous, beans, peas, peanuts, corn chips, snack puffs, popcorn, tofu, seitan, mustard, and many many other foods.