Gluten is a protein found primarily in wheat, which is responsible for the elasticity of kneaded dough which allows it to be leavened, as well as the "chewiness" of baked products like bagels. It can be extracted and cooked to provide a very convincing mock meat, due to its "stringy" texture.
Gluten can be extracted by hand via a time consuming process, or purchased in powdered form from health food stores.
A popular gluten mock-meat product is seitan, which is gluten that's been cooked in a broth to give it a flavor similar to that of beef.
Some people have an allergy to gluten and must go on a gluten-free diet. People with Celiac disease are not able to properly process gluten in their digestive system and must avoid it entirely.
People with autism and autistic spectrum disorders, like Asperger's disorder, may be sensitive to gluten and casein (the protein in milk). See more at gluten-free, casein-free diet.
Gluten is found in wheat and therefore in all products derived from it, including bread, pastry, flour, and many breakfast cereals.
Gluten is used heavily in Chinese Buddhist cuisine.