Gluten, The Enemy.

Gluten is a composite formed from several different proteins. It is found most commonly in wheat and other related grains, such as barley and rye. Adding texture and a characteristic chewiness to baked goods, this ingredient is used in a wide variety of other foods as a thickener and binder, flavor enhancer, and protein supplement. Some people can develop an intolerance to these proteins, however; a gluten-free diet often helps to alleviate symptoms caused by this intolerance and prevents further damage to the body.

Foods Containing Gluten

Along with wheat, other cereal grains that contain gluten are durum, spelt, and einkorn, as well as farro, graham, kamut, and semolina. Many are also used to enhance the flavor and protein of foods, as well as to bind and thicken their consistency. While the grains are often found in breakfast cereals and various baked goods, they can also be included in a variety of other foods that are not as obvious.
Used as a thickener, gluten can be found in soups and broths, as well as gravies and sauces such as ketchup, salad dressings, or marinades. Since it enhances flavor, it is used in bouillon, spice blends, and other foods such as coffees, dairy products, vinegars, and liquors. It can also be found in the substance used to seal envelopes since it acts as a stabilizer.

Gluten is also used on its own as a protein supplement, particularly for people who do not have access to other protein sources. The protein it contains also makes it useful in meat substitutes, especially those used in vegan and vegetarian diets. Imitation meats or wheat meats, such as seitan, are often made of concentrated gluten.
 
Anything that has an ingredient containing the protein (seed) from any edible plant from the grass tribe Triticeae, or anything contaminated during harvesting, transport, or processing of any product (e.g. mainstream brand oats). Go here:
http://en.wikipedia.org/wiki/Triticeae

Gluten Proteins

Within gluten, there are actually four main proteins: albumins, glutelins, globulins, and prolamins. Glutelins and prolamins are found in higher concentrations in wheat, while albumins and globulins are more plentiful in corn and rice. Many people associate wheat with the term "gluten," however, as it is those proteins that are most directly related to health issues such as celiac disease. Glutelins, in particular, are dangerous for those susceptible to intolerance because of the way that acids in the body break them down.
Most of the protein in wheat — 80% — is made up of the prolamin called gliadin and the glutelin called glutenin. When these molecules are joined together due to a chemical reaction, they stretch and harden, allowing dough to form a light, airy loaf with a chewy texture. As a result, these proteins are commonly found in flour and other baking products.