Knowing what metal you are polishing will help you choose which mopscompounds and polishing kits to use. Herewith some clarification.
All metals can be classified as either being Ferrous or Non-ferrous.

Ferrous metals contain iron and sometimes have small amounts of other metals or other elements added to give a required property. They are magnetic and give little resistance to corrosion. These metals are Mild Steel, high speed steel, stainless steels, high tensile steels, high carbon steels, medium caron steels and cast iron.

Non ferrous metals do not contain any iron, are not magnetic and are generally more resistant to corrosion than ferrous metals. Aluminium, Aluminium alloys, copper, brass, lead, zinc, tin and gilding metal.

A pure metal consists of a single element or atom type. Common pure metals are:-aluminium, copper, iron, lead, zinc, tin, silver and gold.

Alloys are a mixture of two or more metals. For example, pure white aluminium is very soft and ductile. Other elements are added to create an aluminium alloy. The aluminium alloy is stronger than Mild Steel, has an improved strength and is resistant to corrosion while still remaining very light.

And there you go. I hope this helps.