The four parts of bread are flour, water, yeast, and salt. Yeast is a microorganism that feeds on starch in flour, converts it to sugar, and expels CO2 and alcohol to make bread rise. Types of yeast include dry active, fresh, instant, and wild. Flours range from lowest to highest protein as cake flour, pastry flour, all-purpose flour, bread flour, and high gluten flour. Salt adds flavor and water gives elasticity and structure. Other ingredients like fat, eggs, and sugar add richness but too much can inhibit yeast.