A market order is an order that a trader enters that does not specify a specific price to execute. A market order will seek liquidity at progressively further prices from the current bid and offer until the order is completely filled. The purpose of the market order is to ensure that the transaction is completed instantly. This can be very useful to traders in a fast moving market because they are assured of the order executing. A market order is the opposite of a limit order, which is an order that must execute at a certain price or better.
A market order can be a buy or a sell order. If the order is a buy order, it will first remove liquidity from the offer. If this does not completely fill the order, the market order will continue to seek liquidity at progressively higher prices until the fill is complete. Conversely, if the order is a market sell order, it will remove liquidity from the bid and progressively lower prices until the order is completely filled.
The obvious benefit of the market order is the speed and surety of execution. A market order will always fill, and it will always fill almost instantly. If a stock is very quickly changing prices and it would be difficult to take the time to enter a specific price a market order can be extremely useful.
This works very well for traders if the order is small, or if there is a lot of liquidity posted in the stock. This type of order can potentially be very dangerous for traders as well. Because price is not specified, the order can legally transact at whatever price is necessary to complete the order. If a trader enters a very large market order relative to the liquidity available in the stock, he risks moving the price significantly away from the current bid and offer. Because high frequency trading programs can remove posted liquidity so quickly, there is always a risk that the programs will remove liquidity before the market order is even able to access it, resulting in an even worse execution for the trader. Traders should always be aware of how large their order size is in relation to the available posted liquidity for a particular stock. All stocks are different in this regard, and the same stock can be different during different days or even during different times of the day. This is yet another reason why traders must always watch the book of a stock while trading.
It should be noted that using a market order in the dark pools can access more liquidity than can be seen on the books. Sometimes this can result in a much better fill for a trader, other times the light pools will pull orders when a large market order is entered into the dark pools. If you have access to a sweep order that will seek both light and dark liquidity it is most beneficial to trader entering the market order.
A market order is simply one more tool in a traders toolbox. It is useful in fast moving markets with ample liquidity. Used at the right time it can be tremendously useful, used at the wrong time it can cost a trader a lot of money. Traders must always be aware of the circumstances when deciding between using a market order or a limit order.