Is there really much of a difference? Can they be used interchangeably?
yes, the flour you use has to fit the product you are making. It's all about the gluten.
I use bread flour for most breads. I want a high gluten content to give the dough strength, elasticity and a good rise.
For some breads, I will use half AP flour and half Bread flour. For cakes and pastry, the flour should be AP or even Cake flour. Most recipes will tell you what they recommend.
Until you know what you are doing, follow the directions. With 'sperience, you can call the shots, since you know where you are going, and understand the chemistry.
