True to its name, all-purpose flour is a good choice for all kinds of baking because it has a middle-of-the-road protein content (between 9 and 13%). Cakes made with cake flour might be more tender, and loaves made with bread flour might rise higher, but the differences are subtle. All-purpose flour is also a good choice for other cooking jobs tackled by flour, such as thickening sauces and coating foods.
4.5 oz. all purpose flour = 1 cup
In a pinch, you can use cake flour (add 2 tablespoons more cake flour per 1 cup all-purpose flour)
Unless a recipe states otherwise, use unbleached all-purpose flour. Bleached flour contains additives that whiten flour and, in some cases, affect its baking properties.
For most accurate measuring, use a scale. Fluff the flour with a fork to avoid densely packed flour. Spoon the flour from the bag into a dry measuring cup and level it with a knife. Never scoop right form the bag, which would compact too much flour into the measuring cup. Don't shake or tap the cup as this packs down the flour, too.
Store flour in an airtight container in a cool, dry place away from direct sunlight.