A stop order is an order placed below the current market price and is triggered if the stock reaches at or below the stop price. Once a stop order is triggered, your order will become a market order. For a NYSE/AMEX/LSE/TSX stock, a stop order is triggered by the last trade. If a sell stop order is placed above the current market price of the stock, it will not be accepted by the exchanges and will be cancelled.
The advantage of the stop order is that you do not need to monitor your stock investments on a daily basis of how your stock is performing. The disadvantage of the stop order is that the stop price can be activated by a short term fluctuation on the stock's price.
Let us consider the example where you buy a stock on the NASDAQ market at $30 per share. If you have an investment strategy where you do not wish to lose much more than 10% of your initial investment, you then place a stop loss order for $27 (after buying the stock). This means that if the stock falls at or below $27 per share, your holding will be sold at the prevailing market price.
Another aspect to keep in mind is that once the stop price is reached, your stop order becomes a market order and the price at which you sell may be much lower from the stop price. In the above example, you may have a market fill at $25 which is more than your anticipated loss of 10%. This is to be considered in a fast moving market where stock prices fluctuate very rapidly.