Types of orders
There are several order types available when you are buying and selling shares.
A market order is the simplest and most common type of order. It is when you tell your broker to buy or sell a stock for you immediately at the best price. Since you do not specify a price, this type of order will almost always be filled, but it may not necessarily correspond to the current market price because the price may have changed by the time the broker executes the order.
You can also place fill/kill orders, limit orders or stop-loss orders.
Find out more about Fill/kill Orders