Normally found in rye, wheat and barley, gluten is the common name for the storage protein in grains. Even with the public eye more interested in the growing fad of a gluten-free diet, many people still don't know what it is. Quite simply, gluten is what makes bread dough, well… doughy.
“The most serious gluten-related problem, celiac disease, affects 1 in 100 people or 3 million Americans, most of which are not diagnosed.” – Mark Hyman, M.D. (Author of The UltraMind Solution: Fix Your Broked Brain by Healing Your Body First and creator of Gaiam's UltraMind Solution Club)
You might be thinking, ‘1 in 100 people isn't as uncommon as I was thinking.' Don't be alarmed. An easy way to find out if you have an intolerance to gluten is after you eat a gluten-rich meal (like pasta and bread). This can be a minor intolerance and produce a bloated or gassy experience, or can be more serious and cause you painful cramps, vomiting, migraines, and even loss of consciousness.
Update from HuffingtonPost.com:
Gluten sensitivity is actually an autoimmune disease that creates inflammation throughout the body, with wide-ranging effects across all organ systems including your brain, heart, joints, digestive tract, and more. It can be the single cause behind many different "diseases." To correct these diseases, you need to treat the cause–which is often gluten sensitivity–not just the symptoms.
Gluten is difficult for our bodies to break down, and some have a harder time. If you're looking to start avoiding this special protein, start eating more fruits and vegetables. That's the first place to start. Then you need to become aware of the nutrition label on your favorite foods. If it's been processed, then you need to read the label to make sure it's not hiding. And the gluten-free market has grown exponentially into widely accessible products in the average grocery store. Have fun and don't beat yourself up. It's easier than you think.