What are the main ingredients in yeast breads and what are their functions?

Flour, Yeast, Water, and Salt. Flour provides structure.  Yeast is a one-celled plant available in dried form, an instant blend, and live cakes. In yeast bread, yeast multiplies and grows by using available sugars and water.  Water helps carry flavorings throughout the product.  Salt strengthens gluten and enhances the flavor.