From Scottrade web page:

How Your Order is Executed

Placing orders can feel like raising kids. You do the best you can to position them to succeed, but once they're out of your hands, they may or may not act the way you expect them to.

Below, we will look at each order type and a few common scenarios in which the order doesn't execute the way you planned. Understanding why orders execute the way they do is essential to placing orders that match your investment strategy.

Market Order Execution

A market order is an order to buy or sell a security at its current price in the market. When you place a market order, you can be confident that your shares will be bought or sold, but you do not necessarily know the exact price.

Limit Order Execution

A limit order allows you to guarantee that, if a trade is possible, you will receive a specified price (limit price) or better.

Stop-on-Quote Order Execution

Stop-on-quote orders allow you to set a price at which you want a market order to be triggered. Your order will not enter the market unless the stop price you specify is met or surpassed, and once it enters the market, it becomes a market order that executes at the next available price.

Stop-Limit-on-Quote Order Execution

A stop-limit-on-quote order is an order to buy or sell a security at a specified price or better (limit price), but only after a given stop price has been reached or passed. The order initially is a stop order that, once triggered, then becomes a limit order. The order will only execute at the limit price or better.

Trailing Stop-on-Quote Order Execution

Trailing stop-on-quote orders allow you to set a stop order at a certain percentage or dollar decimal spread away from the market price. As the market price changes, your stop price changes accordingly.

Conditional Order Execution

Conditional orders allow you to place an order that will only enter the market if your specified condition is met. For example, you could set an order to buy 100 shares of XYZ when it reaches its 52-week low.