Flour explained clearly and simply.

Flour is a fine powder which is the result of grinding cereals such as wheat between stones or metal. Although wheat is the principal source of flour, other cereals such as maize (corn), rice and rye are also used. Flour is the main ingredient of bread and pastry.

When flour is mixed with water the protein 'gluten' is produced this gives dough (water and flour mixed) its elastic like properties. The more gluten contained in the flour the stronger will be the resulting dough. One property of dough that is very important is its ability to contain air bubbles which gives dough it's spongy and light texture when cooked.

There is a huge variety of different varieties of flour suitable for different uses. Most of the common varieties of flour available in supermarkets are sold as fit for a specific purpose (e.g. cake flour, all purpose flour, bread flour), each type of flour having specific properties.

Cake Flour - this is a soft flour which is very low in gluten.
Pastry Flour - this is also a soft flour which has slightly more gluten than cake flour.
Bread Flour - this is a hard flour which is high in gluten and holds it's Shape well when cooked.
All Purpose Plain Flour - a blend of hard and soft flours with medium gluten content.
Self-Raising Flour - this is All Purpose Plain flour which has baking powder and salt mixed in. The baking powder helps the dough to rise when it is cooked. You can make your own using the following ingredients:
100 g plain flour
3 g baking powder
1 g salt

As well as the flours above there are other types of flour available which are made from specific parts of the wheat grain. These are:
Whole Grain Flour - this is made from entire grain.
White Flour - is made from the endosperm part of the grain
Germ Flour - is made from the endosperm and germ part of the grain but excludes the bran part.
Graham Flour - this is a wholewheat flour where the endosperm is finely ground but the bran and germ are coarsely ground. Only commonly found in the USA.

When flour is freshly milled it is naturally a light yellow colour. Over time the flour will turn more white. Commercially though, freshly milled flour is treated with bleaching agents to give it it's pure white colour. These bleaching agents are normally organic peroxides like acetone peroxide or benzoyl peroxide, nitrogen dioxide, or chlorine.

FLOUR FROM OTHER CEREALS
Corn flour - don't confuse this with the English cornflour (which is cornstarch). This is popular in Mexico and southern USA. Masa harina is a type of corn flour which is used to make the traditional tortilla of Mexico.
Durum Flour - used almost exclusively to make noodles and pasta, it is made from durum wheat.
Other sources of flour include rice, chickpea, rye, soy beans and potatoes.

STORING FLOUR
Flour is prone to infestation with moths and beetles so correct storage is essential. Keep flour in a cool, dry, dark place in a container that allows the flour to breath. Avoid airtight containers.

FLOUR CAN BE EXPLOSIVE!
When fine flour is suspended in the air it is truly explosive. There have been countless flour mill explosions over the last hundred years or so killing many people. Although unlikely to happen in your home it is a possibility to be aware of.