Anything with gluten in it, which is wheat, rye, barley, and non-gluten-free oats. But that is easier said than done. Refer to the long answer.
Unfortunately, gluten is hidden in many places. Wheat and barley go by many different names, as well as many different derivatives. In addition, there are many different processed food ingredients that may contain gluten but due to regulations, may not have to state specifically if there is gluten or not. (see: What’s Not So “For Sure”)
Let’s list out the possible names and derivatives of wheat and barley to look for.
Wheat: atta, bulgur, couscous, dinkel (spelt), durum, einkorn, emmer, farina, farro (spelt), fu, graham, hydrolyzed wheat protein, kamut, matzoh (also known by matza, matzo, and matzah), modified wheat starch, orzo, seitan, semolina, spelt, triticale, wheat bran, wheat flour, wheat germ, wheat starch.
Barley: ale, barley flour, pearl barley, beer, brewer’s yeast, lager, malt (including extract, flavoring, milk, syrup, vinegar). Just to note, if malt is listed as an ingredient, it is barley malt, so be sure to avoid it. If the malt is made from any other source, such as corn, it must list that source in the ingredient list (e.g., corn malt.) `