A limit order is placed when the customer buy or sell at a specific price at which his broker will not buy is the stock is priced above the limit or sell if the price is below the limit, however, this kind of order may never be executed (filled). This kind of order is used when the trader wants to control the price rather then the certainty of execution.
In use a buy limit order has to be executed at the limit price or lower. Suppose you don’t want to buy a stock higher then a specific price by placing a “limit order” on the transaction to buy the stock either at that price “or better.” In selling a stock by placing a limit order means that the stock will not be sold at a lower price then what you have specified. Using either type of limit transaction does not mean that your order will be fulfilled. With this type of order you might get fewer shares then you ordered of none at all.
With both buy and sell orders you can have additional constraints placed on the order the most common of which are Fill or Kill (FOK) or All Or None (AON). With the FOK orders they are either filled completely at the first attempt or the order is killed outright and isn’t filled. The AON orders mean that your order must be filled with the specified number of shares. If this type of order is not filled it is still held on the order book to be filled later.