Help Understanding the OSI 7-Layer model!

I am studying for the Network+ certification and I've easily grasped everything so far up until my materials started going over the OSI model and I can't grasp it. I know the layers and everything, but I have problems when it comes to exam questions that ask "What layer does such-and-such protocol fall within?" I'm hoping someone can explain it in a way better than my materials are in terms of the exam. I know that in real-life devices and protocols can be related to multiple layers. How do I know which layers something falls under and what answer they're looking for on the exams?