Flour is grain that has been ground, or milled, into a powder of a desired consistency. In the United States, flour is usually ground wheat, however, it may be almost any grain ground into a light powder.
The most commonly used grains include wheat, rye, barley, rice, spelt (a primitive wheat), kamut (another primitive wheat), and oats. Wheat is the most commonly used bread flour at this time because it provides the best rise due of the quantity and type of gluten in the grain.
If someone specifies "flour" in a recipe, without other qualifications, this is usually a refined wheat flour, such as all-purpose or bread flour. These are called white flours, not because they are made from a white wheat, but because the bran that colors a whole grain flour has been removed. It is worth noting that this has tremendous variation from one part of the United States to another, and even wider variation from one country to another.
French and Italian breads are usually made with flours that have between 7 and 9% protein. Most all purpose flours in the United States have around 10% protein. And bread flours have around 14%. As a result, experimentation with a recipe may be necessary to get the results you want.
To further muddy the waters, the protein content of flour in the United States is measured when the flour has a moisture content of 14%. In parts of Europe the moisture is removed from the flour before the protein content is determined. This means that the protein content of European and American flours are not immediately comparable. When in doubt, do a test bake. Or two.