Short-selling, or simply shorting, is a strategy employed when you believe that the value of some stock will fall. You begin by borrowing a certain number of shares from someone. Then, you sell the shares and buy them back at a lower price-point. Now, you have the same number of shares as before (which you then return to the original owner) as well as cash equivalent to the difference between the price when you borrowed and returned the shares. Shorting is used commonly to bet against a company.