A stock order executed "at the market" is one that is executed at the quickest possible speed with no regard for the price of the stock or security in question. When an investor decides on this type of order, also known as a market order, his stock broker will make the trade at the best price immediately available. This differs from a limit order, in which the broker is ordered to buy or sell only when the stock reaches certain prices. Orders "at the market" offer assurance that they will be executed and have low commission fees attached, but there is a chance that a quick change of price could cost the investor money.
Each potential stock market investor must choose a strategy which suits his or her individual investment needs. Some may prefer to be patient with their trades and commit to them only when the market turns their way. Others may prefer to get in and out of trades quickly and not run the risk of missing out on a potential moneymaking investment. For the latter group, "at the market" trades are usually the better choice since they'll be executed in most cases as quickly as the order is placed.
As an example of an "at the market" trade, imagine a stock that is trading at $30 US Dollars (USD) per share. If an investor wants 10 shares of these immediately, he'll call in a market order to his broker. At that point, the broker then will search for a seller offering these shares at the best available ask price.
The danger of an "at the market" order is that the price of the stock could conceivably change quickly and cost the investor more than originally anticipated. In the example above, if the price rose to $35 USD per share in the time from when the investor checked the price to when the order was executed by the broker, then the investor would be out an extra $5 USD per share for each share bought. If the investor was taking a heavy position on this stock, it could be a costly situation.
In general, stocks that trade in very high volumes are the best choices for "at the market" orders since they generally trade at a price very close to the quoted price. Some stocks that trade at a low volume may be difficult to acquire at the desired price. For those stocks, investors may be better off with a limit order, in which case a broker waits until the price reaches a specified level before making a move. The ability to choose the price may make limit orders advantageous in these situations, even though they command higher commissions from brokers than market orders.