Unlike a stock, which represents equity in a company and can be held for a long time, if not indefinitely, futures contracts have finite lives. They are primarily used for hedging commodity price-fluctuation risks or for taking advantage of price movements, rather than for the buying or selling of the actual cash commodity. The word "contract" is used because a futures contract requires delivery of the commodity in a stated month in the future unless the contract is liquidated before it expires.
The buyer of the futures contract (the party with a long position) agrees on a fixed purchase price to buy the underlying commodity (wheat, gold or T-bills, for example) from the seller at the expiration of the contract. The seller of the futures contract (the party with a short position) agrees to sell the underlying commodity to the buyer at expiration at the fixed sales price. As time passes, the contract's price changes relative to the fixed price at which the trade was initiated. This creates profits or losses for the trader.
In most cases, delivery never takes place. Instead, both the buyer and the seller, acting independently of each other, usually liquidate their long and short positions before the contract expires; the buyer sells futures and the seller buys futures.
Arbitrageurs in the futures markets are constantly watching the relationship between cash and futures in order to exploit such mispricing. If, for example, an arbitrageur realized that gold futures in a certain month were overpriced in relation to the cash gold market and/or interest rates, he would immediately sell those contracts knowing that he could lock in a risk-free profit. Traders on the floor of the exchange would notice the heavy selling activity and react by quickly pushing down the futures price, thus bringing it back into line with the cash market. For this reason, such opportunities are rare and fleeting. Most arbitrage strategies are carried out by traders from large dealer firms. They monitor prices in the cash and futures markets from "upstairs" where they have electronic screens and direct phone lines to place orders on the exchange floor.