An ingredient used in many foods, flour is a fine powder made from grain or other starchy food sources. It is most commonly made from wheat, but also maize (aka corn), rye, barley and rice, amongst many other grasseses and non-grain plants.
Flour is always based on the presence of starches, which are complex carbohydrates.
Usually, the word "flour" used alone refers to wheat flour, which is one of the most important foods in European and American culture. Wheat flour is the main ingredient in most types of breads and pastries. Wheat is so widely used because of an important property: when wheat flour is mixed with water, a complex protein called gluten develops. The gluten development is what gives wheat dough an elastic structure that allows it to be worked in a variety of ways, and which allows the retention of gas bubbles in an intact structure, resulting in a sponge-like texture to the final product. This is highly desired for breads, cakes and other baked products.
Wheat varieties are typically known as "hard" or "soft", depending on gluten content. Hard wheats are high in gluten, and soft wheats are low. Hard flour, or "bread" flour, is high in gluten and so forms a certain toughness which holds its shape well once baked. Soft flour is comparatively low in gluten and so results in a finer texture. Soft flour is usually divided into "cake" flour, which is the lowest in gluten, and "pastry" flour, which has slightly more gluten than cake flour.
\All-purpose flour is a blended wheat flour with an intermediate gluten level which is marketed as an acceptable compromise for most household baking needs.
In terms of the parts of the grain (the grass seed) used in flour -- the endosperm or starchy part, the germ or protein part, and the bran or fiber part -- there are three general types of flour. "White" flour is made from the endosperm only. "Whole grain" flour is made from the entire grain. A "germ" flour may also be made from the endosperm and germ, excluding the bran.
Self-rising flour is "white" wheat flour that is sold premixed with chemical leavening agents. Typical ratios are SA: one cup flour : 1 to 1 1/2 teaspoon baking powder: a pinch to 1/2 teaspoon salt Metric: flour : baking powder : salt
Maize flour is very popular in the southern United States and in Mexico. In the US, "white" corn (maize) flour is usually referred to as cornstarch. "Whole-grain" corn flour is usually referred to as corn meal. Corn meal which has been leached with lye is called masa harina and is used to make tortillas and tamales in Mexican cooking.
100% Rye flour is used to bake the traditional sourdough breads of Germany and Scandinavia. Most rye breads use a mix of rye and wheat flours because rye has a low gluten content.
Rice flour is of great importance in Southeast Asian cuisine. Also edible rice paper can be made from it.
Bread, pasta, crackers, many cakes, amongst many other foods, are made using flour. Wheat flour is also used to make a roux as a base for gravy and sauces. White wheat flour is the traditional base for wallpaper paste. It is also the base for papier-mâché. Cornstarch is a principal ingredient of many puddings.