Ed, I'm no expert, but just an avid student of the Naval aspect of the war, however, my understanding of the word "gunboat" is that it covers all Naval vessels that went armed against the enemy.
As for the difference between frigates and sloops; though both of them were masted vessels (as opposed to the ironclads, etc.), I do believe that the frigates were the much larger masted vessels, and sloops of a smaller class. Someone please correct this statement for the sake of clarity.
What is clear is that the vessels of the USN were classified according to class - class 1 being of the largest variety, and then going down in size and personnel from there. The Naval Registers of each year show the vessels within each class.