At its most basic, plain flour is flour that does not contain a raising agent.
An ingredient used in many foods, flour is a fine powder made from cereals or other starchy food sources. It is most commonly made from wheat, but also maize (a.k.a. corn), rye, barley and rice, amongst many other grasses and non-grain plants (including many Australian species of acacia). Flour is the key ingredient of bread, which is the staple food in many countries, and therefore the availability of adequate supplies of flour has often been a major economic and political issue. Flour can also be made from legumes and nuts, such as soy, peanuts, almonds, and other tree nuts.
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.