Bread flour is a high-protein flour, specially formulated for making yeast breads. The combination of extra protein, a tiny bit of malted barley to help the yeast, and vitamin C or potassium bromate to help the formation of the gluten, helps the dough rise and retain gasses as it bakes, resulting in greater volume and better texture.

You can certainly substitute all-purpose flour in a recipe, but the results will be less spectacular. If you have a box of wheat gluten handy, you can add a tablespoon or so to your bread recipe to get essentially the same results as if you had used the bread flour.