Shorting is used to capitalize on a drop in a stocks price rather then a rise in price. Buy a stock...goes up you make money. Short (sell) a stock...goes down you make money.

But how do I sell a stock that I do not own you may ask. You borrow the stock from your broker and sell it to someone else.

Your broker has it in inventory or they borrow it from another brokerage firm. They actually loan you the stock to sell to someone else. This is all done automatically and instantly when you place an order to short a stock...