Wheat flour is made in a wide range of varieties, depending on its intended use. The differences in the varieties depend on the type of wheat it is made from, how finely it is ground, and the protein content (since protein is what creates gluten). Most of the recipes in this book will be made with all-purpose flour, with a few exceptions.
For home bakers, I recommend keeping three types of flour on hand: all-purpose flour (preferably unbleached), bread flour, and cake flour. You might also want to add whole wheat flour to your lineup if you are interested in whole-grain baking.