There are two types of metals for investment: precious metals as opposed to base metals. Precious metals include gold, silver, platinum, and base/industrial metals include copper, nickel, aluminum, zinc, lead, and iron/steel.  The reasons for investing in precious metals and base metals can be very different, but their prices are correlated because of inflation.

Although metals have a tendency towards fluctuation on price, investors feel safe knowing that at least a portion of their portfolio is in precious metals and, as such, they are not entirely dependent on the financial markets or currency.  

Most precious metals can be purchased in the form of the actual asset.  As an alternative, they can be purchased in the form of a contract giving one the right to buy the metal in a certain quantity.  Some also invest through mining operations, which are considered to be valued in a similar manner.

A stronger dollar makes base metals, which are denominated in US currency, more expensive for holders of other currencies.