What is Short Sale

A short sale is generally the sale of a stock you do not own. Investors who sell short believe the price of the stock will fall. If the price drops, you can buy the stock at the lower price and make a profit. If the price of the stock rises and you buy it back later at the higher price, you will incur a loss.

When you sell short, your brokerage firm loans you the stock. The stock you borrow comes from either the firm’s own inventory, the margin account of another of the firm’s clients, or another brokerage firm. As with buying stock on margin, you are subject to the margin rules. Other fees and charges may apply. If the stock you borrow pays a dividend, you must pay the dividend to the person or firm making the loan.

Fuente: U.S. Securities and Exchange Commission
http://www.sec.gov/answers/shortsale.htm