what is 'SHORT SELLING" ?

In finance, short selling or "shorting" is a way to profit from the decline in price of a security, such as stock or a bond.
Some investors "go long" on an investment, hoping that price will rise. To profit from the stock price going down, short sellers can borrow a security and sell it, expecting that it will decrease in value so that they can buy it back at a lower price and keep the difference. The short seller owes his broker, who usually in turn has borrowed the shares from some other investor who is holding his shares long; the broker itself seldom actually purchases the shares to lend to the short seller
For example, assume that shares in XYZ Company currently sell for $10 per share. A short seller would borrow 100 shares of XYZ Company, and then immediately sell those shares for a total of $1000. If the price of XYZ shares later falls to $8 per share, the short seller would then buy 100 shares back for $800, return the shares to their original owner, and make a $200 profit. This practice has the potential for an unlimited loss. For example, if the shares of XYZ that one borrowed and sold in fact went up to $25, the short seller would have to buy back all the shares at $2500, losing $1500.
However, the term "short selling" or "being short" is often used as a blanket term for all those strategies which allow an investor to gain from the decline in price of a security. Those strategies include buying options known as puts. A put option consists of the right to sell an asset at a given price; thus the owner of the option benefits when the market price of the asset falls. Similarly, a short position in a futures contract, or to be short a futures contract, means the holder of the position has the obligation to sell the underlying asset at a later date.
In fact, what is many times labeled short selling is options or futures activity, since this activity greatly magnifies the gain that results from a securities price loss. For example, if the next earnings release of XYZ company is going to show that its profits declined somewhat in some of its divisions, its stock might decline only 5 percent when that information is released. Someone within the company who wants to trade in inside information however would probably not be satisfied with only a 5 percent gain on his short sell and instead would buy put options or other derivatives or futures to gain possibly 20 or more percent on the decline in the stock price of XYZ.