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. Bread flour is called for in many bread and pizza crust recipes where you want the loftiness or chewiness that the extra gluten provides. It is especially useful as a component in rye, barley and other mixed-grain breads, where the added lift of the bread flour is necessary to boost the other grains.
All-purpose flour is made from a blend of high- and low-gluten wheats, and has a bit less protein than bread flour — 11% or 12% vs. 13% or 14%. You can always substitute all-purpose flour for bread flour, although your results may not be as glorious as you had hoped. There are many recipes, however, where the use of bread flour in place of all-purpose will produce a tough, chewy, disappointing result. Cakes, for instance, are often made with all-purpose flour, but would not be nearly as good made with bread flour.