The Earth's crust contains metals and metal compounds such as gold, iron oxide and aluminium oxide, but when found in the Earth these are often mixed with other substances. To be useful, the metals have to be extracted from whatever they are mixed with. A metal ore is a rock containing a metal (in elemant form or as a compound) in a high enough concentration to make it worthwhile extracting the metal.
The method used to extract metals from the ore depends on their reactivity.
Thus the method of extraction of a metal from its ore depends on the metal's position in the reactivity series:
Reactive metals such as aluminium are extracted by electrolysis, while less-reactive metals such as iron may be extracted by reduction with carbon or carbon monoxide.
Note that gold, because it is so unreactive, is found as the native metal and not as a compound, so it does not need to be chemically separated. However, chemical reactions may be needed to remove other elements that might contaminate the metal.
[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science