A limit order is condition that must be met before the trade is executed. For instance, if you are buying GOOG with a limit price of $300, your order will not be filled untill the asking price of GOOG is at $300 or better. Think of the limit price as the highest price you are willing to pay. If the stock is trading lower than your limit price, then you will get the lower price (you are not obligated to pay your limit price).