Bread flour contains 13 or more grams of gluten-forming protein per cup. By contrast, the all-purpose flour we generally use for baking contains anywhere between 9 grams and 11 grams these same proteins. This means that doughs made with bread flour will be stronger and more elastic, and the breads they make will be more dense and chewy.
This kind of flour is best used in recipes where you want that chewier texture. Pizza dough, artisan round loaves, bagels, and soft pretzels are all good candidates. You can also use this high-gluten flour when combining with low-gluten or gluten-free flours like rye, whole wheat, and buckwheat for better structure. I've also found it useful to use bread flour in recipes with a lot of "stuff" in the dough, like seeds and dried fruit.
If you're substituting bread flour in a recipe that calls for another kind, know that bread flour will absorb more water than other kinds of flour. You'll want to increase the liquids in the recipe by a quarter or half cup to compensate. You can also mimic the strengthening effect of bread flour by adding vital wheat gluten. Between 1 and 4 tablespoons of vital wheat gluten per loaf is all you need.
If you're feeling extra-fancy, look for a bag of Italian "00" flour. This is a high protein flour similar to bread flour, but ground from a different kind of wheat. It makes breads with a strong gluten structure, but with less chewiness in the final bread. It's used most frequently for making pasta and authentic Neapolitan-style pizzas.
The best source I've found for bread flour is from King Arthur Flour, available at most grocery stores and online. This flour has a consistent quality and protein content, and bakes lovely loaves week after week:
Find It! King Arthur Unbleached Bread Flour, $4.95 for a 5-pound bag
Do you bake with bread flour?
(Image: King Arthur Flour)