Flour is a powder that is made by grinding cereal grains, like wheat, rice, and corn. The most common grain used to produce flour in the US is wheat. There are many types of wheat plants and these plants impart different properties to the flour based on their starch and protein content. The three main types of flour are bread, all-purpose, and cake flour, but there are other variations of these flours which have gone through other processing or additions, such as self-rising flour.
The protein content of flour determines how it will function in a recipe. High protein flours like bread flour are made from hard red spring wheat kernels and have the ability to form much more gluten and hold more water than other types of flour. Two cups of bread flour can hold one cup of water. Bread flour typically has a protein content of about 12%.
All-purpose flour is the most common type of flour found in the US because of its intermediate level of protein. It is a blend of soft and hard wheat grains giving it the properties of both. AP flour can be used to make cakes, breads, cookies, and most other baked goods. It has a protein content of 10.5%.
Of the three main types of flour, cake flour has the lowest protein content (8%). The gluten structure it forms is much weaker and less elastic than those formed from the other types of flours. It has the ability to create very light crumbs and textures making it ideal for cake making. It requires 2 ¾ cup of cake flour to hold one cup of water. In recipes that call for cake flour, it is possible to substitute for AP flour by subtracting 2 tbs from every cup of AP flour used. The final product will not have the same texture, however.
Flour can also be used as a thickening agent due to its ability to undergo gelatinization. The starch molecules release amylose and amylopectin to the liquid and become engorged by the liquid once a certain temperature is used. Flours with lower amounts of protein are best for thickening. Flour is commonly used to thicken sauces, gravies, and pie fillings.