Futures are contracts whose value is based on the change in price of an underlying commodity. The contract specifies a future date at which the investor can buy a set amount of the the commondity at a set quality. Obviously, the largest commodity market is for oil since its worldwide demand is also the highest.
Traditionally, commoditiy markets were used by farmers to hedge (or lock in) the price of future crops or livestock based on the current price so that they would not be subjected to huge losses based on the shifting demands of the market or weather that could ruin crops.
The futures market became a financial powerhouse for all variety of investors after the commodity boom in prices and interest that began in 2001. The Chicago Mercantile Exchange (CME) is the world's largest futures market and has been in existence since 1919.