Typically, the short-seller will "borrow" or "rent" the securities to be sold, and later repurchase identical securities for return to the lender. If the security price falls, the short-seller profits from having sold the borrowed securities for more than he later pays for them. However, if the security price rises, the short seller loses by having sold them for less than the price at which he later has to buy them... Shorting stocks relies on the fact that securities are interchangeable, so that the securities returned do not need to be the same securities (i.e. the same physical pieces of paper) as were originally borrowed. Source:
http://www.hedgefundfaq.com/answer.php?q=short -selling