What is bread flour, and what is it used for?

Bread flour is defined by its protein content.  It is 11-13% protein. It needs the protein to form gluten (a complex elastic protein formed when smaller flour proteins are hydrated and kneaded) in order to hold the gas (CO2) produced by the yeast in the dough when the dough is rising and baking.  It can be bleached or unbleached and is usually enriched.   In addition, ascorbic acid (vitamin C) can be added as a “dough conditioner” to help strengthen the protein matrix.
 
Source(s):
nal.usda.gov.  “Search the USDA National Nutrient Database for Standard Reference.”
edocket.access.gpo.gov. Food “ Code of Federal Regulations. 403 PART 137.  Cereal Flours And Related Products Subpart A”
wheatfoods.gov.  “Difference in Wheat Flours.”
ams.usda.gov. “Type I - Bread flour (Hard wheat flour) Type II - Bread flour (Bakers hard wheat Hearth-style)”