Observing Passover requires a certain amount of knowledge when it comes to kosher do's and don'ts. In addition to eating matzah during your seder, Jews are prohibited from eating leavened bread during the entire week of Passover. A number of specific foods are also off limits.
This article will provide a brief overview of what foods should be avoided during Passover, but should not be taken as a definitive guide - if you have specific questions about Passover kashrut, it's always best to check with your rabbi.
In addition to avoiding leavened bread, Jews are also supposed to avoid foods made with wheat, barley, rye, spelt or oats - unless those foods are labeled "kosher for Passover." The reason these grains can be eaten under some circumstances is that the rabbis determined foods containing these grains must be cooked in 18 minutes or less in order to be considered kosher for Passover. They felt that was just enough time to prevent any natural leavening in the grains from making food rise. "Kosher for Passover" foods are made with flour that is specifically prepared for Passover consumption and are usually made under the supervision of a rabbi.
All five of these forbidden grains are collectively called "chametz." (Pronounced ha-mets.)
In the Ashkenazi tradition there are additional foods that are usually forbidden during Passover. These foods are called "kitniot" (pronounced kit-neeh-oat) and include: rice, millet, corn and legumes like beans and lentils. These foods are off limits because the rabbis determined they violated the principle of ma'arit ayin. This principle means that Jews should avoid even the appearance of impropriety. In the case of Passover, kitniot can be ground up and cooked with like flour, so they should be avoided.
In Sephardic communities kitniot are eaten during Passover. It is also not uncommon for vegetarians who identify as Ashkenazi Jews to follow the Sephardic tradition during Passover - it's tough to be a vegetarian during Passover if chametz and kitniot are off the table!
Other Passover Food Tips
Walk down the "kosher for Passover" aisle at the supermarket and you'll likely find a number of foods you didn't think would come under Passover food guidelines. For instance: sodas, coffee, some kinds of alcohol and vinegar. These foods are often made with chametz or kitniot at some point during the production process. Just think about how many foods contain corn syrup!