A stroll down the supermarket's baking aisle reminds me that there are more than a few kinds of flour to choose from. To decide which type is best for the kind of baking you do, it helps to understand that flour is made up of carbohydrates (or starch), proteins, and in the case of whole-wheat flour, a bit of fat. Of these three nutrients, protein matters most to the baker. The proteins in wheat are called gluten-forming proteins, and the quantity and quality of these proteins determines how a flour will perform in the kitchen.
A high percentage of protein means a harder (stronger) flour best suited to chewy, crusty breads and other yeast-risen products. Less protein means a softer flour, best for tender and chemically leavened baked goods, like pie crusts, cakes, cookies, and biscuits.
Since the protein content of wheat can range from 5% to 15%, the flour industry has established labeling standards that help us find the right flour for our needs.