They are the same thing, just more marketing noise. All basic rings are cast iron. In more racey performance rings you can improve on that, but for what 80% of us would use, it's an iron ring, that has a coating on the face of the top ring (where the ring meets the cylinder wall). That coating might be chrome plating (very hard, not for a street or drag race engine, really) molybdenum (called moly for short), or plain iron faced. All take a unique cylinder finish, so your machinist should tell you what the cylinder is honed for, and you buy that, and follow that prescribed ring seating process.
