Bread flour is higher in protein than all-purpose flour, which may result in a heavier texture. In fact, one cup of bread flour even weighs more than one cup of all purpose flour. For best results, use bread flour only when the recipe specifically calls for it. For example, this recipe for Zucchini Bread with Moroccan Spices calls for both bread flour and whole wheat flour.
For more information on different types of flours, see Flour Power: A Guide to Using Alternative Flours.