Gluten is something that must be avoided by those with coeliac (Yanks know it as celiac) disease due to the severe reactions upon ingestion. People with wheat allergy also need to avoid or limit gluten.
Outside of this disease, gluten has become a new food woo, with fad diets popping up touting themselves gluten-free; including products that are not even grain such as gluten-free salt. This is usually done because people will pay larger amounts of cash for things perceived as better for you. These products have arisen due to the mistaken belief that gluten is somehow responsible for many bowel problems in non-coeliacs and that better health can be achieved by abstaining from it. Some quacks have gone so far as to "link" gluten to a myriad of diseases such as autism in a similar way that quacks "linked" vaccines to autism.
It's generally understood that a lot of people who support gluten-free diets for non-celiacs don't quite understand what they're talking about. Gluten is the product of the combination of two storage proteins in wheat and related grasses, glutenin and gliadin. Gliadin (and occasionally analogous proteins in other grains, called prolamins as they're rich in proline and glutamine) is the offender; in certain people with a genetic predisposition, its metabolites can trigger a false alarm immune response, attacking the small intestine and causing the symptoms of celiac.