Many people think that all metals are just that, metals. But there is a difference. Take copper, for example. Copper is mined throughout the world and appears on the Periodic Table of Elements. This means that copper is a metal by itself and does not need anything else to help make it copper.
On the flip side, Brass is not a metal but an alloy. Brass has multiple metals inside of it, including copper, zinc, tin, aluminum, nickel, and sometimes iron or manganese. All the listed metals appear on the Periodic Table and thus are called metals. But brass has all of those metals inside of it (NOTE: There are many different types of brass for different applications).