At any given point, only a certain amount of a publicly traded company’s stock is floating freely in the market. The rest is held in various portfolios, funds, and investment vehicles. Now, everyone’s familiar with the basic idea behind the stock market: you buy stock when it costs little, and you sell it when it costs a lot, profiting on the difference.
But that assumes a company’s value is going to increase. What if, instead of betting a company will go up, you want to make money betting the company will go down? You can — by selling stock you don’t own.
Say you borrow a certain amount of stock from someone who already owns it. You pay a fixed fee for borrowing the stock, and you sign a contract saying you will return exactly the same amount of stock you took after some amount of time. So, you might borrow a thousand shares of Apple stock from me (I don’t actually own any, but play along), pay me $100 for the privilege, and sign an obligation to return my stock in 3 months. At the time, Apple stock is worth $10 per share.
After you borrow the stock, you immediately sell it. At $10 a share, you get $10,000. Two and a half months later, another rumor about Steve Jobs’ health sends AAPL crashing to only $6 per share for a few hours, so you buy a thousand shares, costing you $6,000. You give me back those shares. Because you successfully bet the company would go down in value, you earned $4,000 minus the borrowing fee. This is called short-selling or shorting the stock, and the downside is obvious: if your bet was wrong, you would have lost money buying back the shares that you have to return to your lender.