Answer: The way most investors look to make money in stocks is to buy the stock, hold it for a time, and sell at a profit. But what if a stock is trading at what you believe to be too high a price? You then might consider a short sale. In a short sale you actually sell the stock first, borrowing the shares from a broker with a promise to return those shares at a later date. If all goes well for you, the stock will fall in price and you will buy the shares back at that lower price, return them to the broker, and pocket a profit. This may sound simple, but your risk is far greater being on the short side. As a buyer of a stock, I can make 10 or 20 times my money on the right stock pick (think Microsoft), but if you are short, that mean you can lose far more than the initial cost of the stock. As it goes up, you might decide to panic and buy it back, this is called a 'short squeeze' where many short sellers cover their positions to limit their losses.
When shorting stocks, remember this ditty;

"He who sells what isn't his'n, must buy it back or go to prison."