Gluten is a protein found in most grains. The gluten found in wheat, barley, rye and triticale (a cross between wheat and rye) can trigger serious health problems or other insensitivities. While other grains such as corn, rice and quinoa also contain gluten, they don't seem to cause the same problems as wheat, barley, rye and triticale.
A gluten-free diet is also popular among people who haven't been diagnosed with a gluten-related medical condition. The claimed benefits of the diet are improved health, weight loss and increased energy, but more research is needed.