All-purpose flour is a powder made from a blend of hard and soft wheat, which provides a range of 8-11% protein. It has the bran and germ removed, leaving only the starchy endosperm.

All  purpose flour is, as the name suggests, a strong "pantry" flour for its ability to be used in an all around fashion. Cake, pastry, bread and cookies all benefit from the gluten AP flour can develop this providing good structure for all.

Wheat flour is composed of moisture (12%), protein (10-12%), fat (1%), carbohydrates (75%), and ash (0.4%). All-purpose is typically around 10% protein but will depend on the supplier’s current specifications.

Cook's Illustrated has developed the following information as Brand guidlenes:

"All-purpose flour manufacturers don’t typically list the protein count on the label, but we’ve gathered that information and used it to group the major brands into three categories: high, moderate, and low protein. Use this list to choose a flour when your recipe calls for all-purpose flour with a specific protein count."

High Protein

• King Arthur Unbleached Enriched All-Purpose Flour: 11.7% protein
• Heckers/Ceresota Unbleached Enriched Presifted All-Purpose Flour: 11.4-11.8% protein

Moderate Protein

• Gold Medal Unbleached All-Purpose Flour: 10.5% protein
• Gold Medal Bleached All-Purpose Flour: 10.5% protein
• Pillsbury Unbleached Enriched All-Purpose Flour: 10-11% protein
• Pillsbury Bleached Enriched All-Purpose Flour: 10-11% protein
• Hodgson Mill Unbleached All-Purpose Flour: 9.5-10.5% protein
• Martha White Enriched Bleached Pre-Sifted All-Purpose Flour: 9-11.25% protein

Low Protein

• White Lily Enriched Bleached Plain All-Purpose Flour: 7-8.5% protein

They further comment:

"Two All-Purpose Flours for All Your Baking Needs

So, do you need to stock all three types of all-purpose flour in your pantry? No, but we do suggest that you keep two types on hand: moderate-protein all-purpose flour and high-protein all-purpose flour.

For a moderate-protein all-purpose flour, we particularly like and Gold Medal, which has 10.5 percent protein; for a high-protein choice, we recommend King Arthur, which has 11.7 percent protein. Both manufacturers mill to a specific protein count, instead of a range, which can make your baking results more consistent. We recommend a moderate-protein all-purpose flour for the vast majority of baked goods, but for baked goods that require a little more gluten development, a high-protein all-purpose flour is preferable.

We almost never call for low-protein all-purpose flour. If we are making something like an ultratender cake, we turn to cake flour, which has a similarly low amount of protein."

Bleached and Unbleached

Bleaching is a chemical process used to make the flour whiter in appearance and to improve it’s functional properties. Freshly milled flour is not white, but bleaching does not yield results that are as good as aged flour. Aging serves to naturally bleach the flour through oxygenation, which also serves to improve the gluten. However, this is a costly process, involving a great deal of storage space, labor, and a high risk of pest infestation. Chemical bleaching, such as with chlorine gas or benzoyl peroxide, is much less expensive. This bleaching lightens the flour and also improves the strength and elasticity of the gluten by increasing the number of disulfide bonds.

Enriched

White flour is required by the FDA to be enriched with thiamin, riboflavin, niacin, folic acid, and iron.