What is Gluten Free?

I have a friend with Celiac disease.  The Reader's Digest version explanation is that her small intestines do not grab the nutrients from food passing through.  She is gluten intolerant. That means she has to avoid foods that contain gluten.  Gluten is proteins found in wheat, rye, barley and spelt.  After years of living with celiac she has learned that she can eat some wheat products cautiously.  I also know a person who had many of the symtoms listed that pointed to celiac but responsibly went to a doctor for testing to find out she was not sensitive to gluten.