What Is a Stop-loss Order?
It is an order placed with a broker to buy or sell once the stock reaches a certain price.
...Another thing to keep in mind is that once your stop price is reached, your stop order becomes a market order and the price at which you sell may be much different from the stop price. This is especially true in a fast-moving market where stock prices can change rapidly.
Source Investopedia: The Stop-Loss Order - Make Sure You Use It
Whenever your stop loss order price is reached, it is executed as a market order. If you want to buy at a certain price, there has to be a seller on the other side to complete that transaction. If there is no buyer or seller at that specific price for the other side of the transaction, then the market order is filled at the next available price. If your order is gapped over during an illiquid market period such as weekend open or NFP, then you will likely experience slippage. If you're trading in the forex, stock, or futures market, then you have to be willing to accept the risk that goes along with having orders executed during illiquid market conditions. There's no sugar coating this.
Slippage is one of the risks you assume when trading. The risk of slippage is greatest during volatile markets such as around a news announcement like NFP, or when holding trades over the weekend. Speaking of NFP, take a look at the below chart which shows the GBP/USD tick chart right at 8:30am ET during the March 8, 2013 NFP release.
http://img812.imageshack.us/img812/2...01332723pm.png
GBP/USD dropped from 1.50361 to 1.49972 in one tick. Yes one tick! That means there were no quotes quoted by the liquidity providers in between those prices. If you had an order to sell GBP/USD at 1.5020, your order would not have been filled at 1.5020 because there was no quote at that price from a liquidity provider offering to buy when you wanted to sell. You likely would have been filled at the next available price around 1.49972. FXCM isn't deciding to fill or not fill at certain prices, your orders are filled based on liquidity and where there is liquidity on the other side of the transaction to fill your order.
Great questions! For someone who is "justlearning", I must say you are quick study. My compliments