Gluten

Wheat and other cereals that are made into flour contain proteins, one of which is glutenin, commonly known as gluten. Viewed alone, gluten is a tough, elastic, grayish substance resembling chewing gum. It's the gluten in flour that, when dough is kneaded, helps hold in the gas bubbles formed by the leavening agent. Gas contained within a dough or batter helps a bread or other baked good rise, creating a light structure. Most (but not all) flours contain gluten in varying amounts. Bread (or hard wheat) flour has a high gluten content and is therefore good for yeast breads, which require an elastic framework. On the other hand, low-protein (and therefore low-gluten) cake flour has a softer, less elastic quality and is better suited for cakes.