Bread flour is a high-gluten flour that has very small amounts of malted barley flour and vitamin C or potassium bromate added. The barley flour helps the yeast work, and the other additive increases the elasticity of the gluten and its ability to retain gas as the dough rises and bakes.
Flour sold as high-gluten or simply gluten flour has been treated to remove most of its starch, which leaves it with proportionately more of the proteins that produce gluten. It is generally used as an additive to doughs made of low-gluten flours, such as rye flour, to give them the elasticity that they can’t muster on their own.
Some people use high-gluten flour to make a low-calorie loaf of bread, but, because high-gluten flour is about eight times as expensive as bread flour, most people don’t make the substitution.