The flour is a food product from the milling of the cereals, usually wheat or corn but also barley, spelled, rice, oats, rye, kamut and many others.
When in a recipe, we read the word “flour” we’d wonder which kind of flour the recipe is talking about. If no type is specified, usually would be wheat flour type “00”, flour for all purposes. Pay attention when buying flour according to purpose, recipes and quality.  Main property of wheat flour is the presence of two proteins, gliadin and glutein, that mixed with water, form gluten. Gluten is a complex protein able to make the mass of flour and water compact, elastic and capable of retaining gases that develop inside creating bubbles. Therefore gluten is the proteine able to give the characteristic spongy texture to bread and other leavened foods . Depending on gluten quantity, the flour will be more or less strong, and the mixture with the water will be more or less elastic and resistant and will react in a different manner with the yeast. This characteristic is usually indicated on the industrial packaging with the letter “W”.Greater will be the content of W, greater will be the amount of gluten and the flour will be stronger. Weak flours are mainly used to bake biscuits and little pastry; medium strenght flours usually used  for French bread or pizza, strong flour for cakes souffle, pizza, bread.Special flours, such as American or Canadian are usually used in combination with other weaker flours to give strength to the dough. In order to discover the factor W even when it is not indicated in the box you can refer to the nutritional table: high protein percentage usually means higher strength of the flour.