Ingredients that contain gluten are derived from,
grown with, or processed with wheat, barley, and rye.
Please note that wheat, barley, and rye have different names and variants that may not clearly indicate you are about to ingest gluten. For instance, spelt and einkorn are variants of wheat, malt vinegar is derived from barley, and triticale is a hybrid of wheat and rye.
To make matters more complicated, there exist a variety of questionable or controversial ingredients that may contain gluten depending on how they are sourced or processed. For instance, caramel color is generally considered safe in the United States but is often sourced from wheat internationally.
Another example is oats, which are often grown in shared fields with wheat and/or are processed with wheat products during manufacturing. It is commonly recommended that people on a gluten-free diet purchase certified gluten-free oat products.
Ingredients like maltodextrin or modified food starch may be sourced from wheat and should be labeled accordingly; however, if you are extremely sensitive to gluten, you may react to even minimal amounts of gluten found in these ingredients.
Products derived from wheat, barley, or rye that are highly processed and/or distilled are discussed in the Alcoholic Beverages post.
Always check nutrition labels and contact the manufacturer as needed to discuss your concerns or ask questions about the product(s) available, manufacturing policies, etc.
So, what can you eat?
This is by far the question asked most by people new to gluten-free living. The good news is that there are a variety of foods that are naturally safe* and can be found at any grocery store.
Fruits, vegetables, meat, poultry, fish and seafood, dairy, beans, legumes, and nuts are all gluten-free in their natural state. You can still eat a variety of grains and starchy foods (like corn, rice, and potatoes), although it is not recommended to purchase grains or other food items from bulk bins due to high risk of gluten contamination.
*All foods listed as safe can be affected by processing and growing conditions, so it is important to read all available nutrition labels and/or ingredient lists to confirm they are gluten-free.