Gluten is a special type of protein that is commonly found in rye, wheat and barley. As a result it is found in most types of cereals and in many types of bread. Gluten is actually formed by the bonding of two other proteins, gliadin and glutenin.
Not all grains contain gluten. Some grains that do not contain gluten include wild rice, corn, quinoa and oats.
Gluten can be removed from wheat flour, producing wheat starch. However, it is virtually impossible to remove ALL of the gluten. Even food labelled as gluten-free contains trace amounts of gluten.
Gluten helps make your bread elastic, providing that nice, chewy texture it has when eaten. For this reason, flour that has had most of its gluten removed, produces a sticky dough that feels a bit like chewing gum.
Gluten provides many additional important qualities to bread. For example, gluten produces a mesh that helps to trap the gases that are released during fermentation in the dough, so the bread is able to rise before it is baked. Gluten also firms up when cooked and helps ensure the bread maintains its shape. The downside of this is that gluten is believed to be partly responsible for causing bread to become stale (along with starch granules).
Gluten is also able to absorb liquid, which is why bread is capable of soaking up broths and soups.