Fragging glue is great for when you need to stick coral to plugs or rocks. Coral Glue can also be used to secure live rock and other items in the aquarium. Be sure when fragging corals and gluing to the plug that you use the minimal amount required, generally it does not take much coral glue or epoxy to adhere the coral frag.
Epoxy is used generally for securing live rock or base rock together to make rock formations. It also works great for securing coral frags and colonies to your main rock structure in your aquarium.