An option contract giving the owner the right, but not the obligation, to sell a specified amount of an underlying security at a specified price within a specified time. This is the opposite of a call option, which gives the holder the right to buy shares.
Investopedia Says:
A put becomes more valuable as the price of the underlying stock depreciates relative to the strike price. For example, if you have one Mar 08 Taser 10 put, you have the right to sell 100 shares of Taser at $10 until March 2008 (usually the third Friday of the month). If shares of Taser fall to $5 and you exercise the option, you can purchase 100 shares of Taser for $5 in the market and sell the shares to the option's writer for $10 each, which means you make $500 (100 x ($10-$5)) on the put option. Note that the maximum amount of potential proft in this example ignores the premium paid to obtain the put option.
