Gluten is a group of proteins found in grains such as wheat, barley and rye as well as oats but in a smaller amount. Spelt, Kamut, Emmer, Farro and Durum Wheat are all different types of wheat which all contain gluten too. Bulgar Wheat, Cous Cous, Freekeh and Semolina are types of Durum Wheat. Triticale is a cross-breed of wheat and rye and also contains gluten.
The main components of the gluten proteins are gliadin and glutenin. In oats similar proteins are called avenins. The percentage of avenins in oats is significantly less than the percentage of gluten proteins in wheat, barley and rye and there are structural differences. To learn more about oat avenins read Are oats gluten free?
When flour containing gluten is mixed with water to make a dough, these proteins make it sticky and elastic. This elasticity traps the air bubbles which contributes to the ability of bread or similar baked goods to rise when heated. This elasticity also gives the baked goods a bouncy texture and helps to retain moisture.
Gluten is also used to improve the texture and stability of processed foods such as processed meats, sweets, ice cream, marinades, sauces and dressings.