The term Base Metals refers to the group of industrial metals that are traded in the open market that exclude the precious metals.
Whereas precious metals include Gold, Silver, and Platinum (metals difficult to mine, and therefore considered rare) Base Metals include the more common mined metals that are used in every day industry building and manufacturing. This being said, their prices go up and down with the health of the world economy, with prices falling during recessionary times and climbing during growth periods. This would include Copper, Nickel, Lead, Zinc, and Tin. While both precious and base metals are traded on exchanges, base metals are often times traded in bulk unit sizes, such as tones, where precious metals are usually measured and quoted in price per ounce.