What is Flour?
In the culinary sense, flour is a powder made of cereal grains, other seeds, or roots.

Types of Flour
Below are some of the most common types of flour with small descriptions.  (www.about.com)

Wheat Flour
Wheat flour is the most common flour used in baking. There are different types of wheat flour, and they're distinguished by the amount of gluten they contain.

Gluten is the wheat's natural protein, and it's what gives baked goods their structure. When dough is kneaded, these glutens develop and become firm. Flours made from hard, high-protein varieties of wheat are called strong flours. They have a higher gluten content. Flours made from softer, low-protein wheats are called weak flours, and are lower in gluten.

All-Purpose Flour
All-purpose flour is formulated to have a medium gluten content of around 12 percent or so. This makes it a good middle-of-the-road flour that can be used for a whole range of baking, from crusty breads to fine cakes and pastries. Even so, most professional bakers don't use all-purpose flour but instead use either bread flour, cake flour or pastry flour, depending on what they are baking.

One sifted cup of all-purpose flour should weigh around 4½ ounces or 125 grams.

Bread Flour
Bread flour is a strong flour, meaning that it has a relatively high gluten content — usually around 13 to 14 percent. A handful of bread flour will feel coarse and will look slightly off-white. Bread flour is used for making crusty breads and rolls, pizza doughs and similar products.

One cup of bread flour will weigh around 5 ounces or 140 grams.

Cake Flour
Cake flour is made from soft wheat and has a lower gluten content — around 7½ to 9 percent. Its grains are visibly finer than bread flour, and it is much whiter in color. Its fine, soft texture makes it preferable for tender cakes and pastries.

One sifted cup of cake flour will weigh around 3½ ounces or 99 grams.

Pastry Flour
Pastry flour is slightly stronger than cake flour, at around 9 to 10 percent gluten. It can be used for biscuits, muffins, cookies, pie doughs and softer yeast doughs. It has a slightly more off-white color than cake flour.

One sifted cup of pastry flour will weigh just over 3½ ounces, or around 101 grams.

Self-Rising Flour
Self-rising flour is a peculiar bird. It's basically ordinary all-purpose flour that has baking powder and salt added to it. Intended as a convenience, it's really anything but — the main problem being that there's no way to control how much baking powder it contains. Also, when stored in humid climates, the baking powder in the flour will quickly lose its effectiveness, making things even more unpredictable. Unless you have no other options, this type of flour is probably best avoided.