Best Execution
In deciding how to execute orders, your broker has a duty to seek the best execution that is reasonably available for its customers' orders. That means your broker must evaluate the orders it receives from all customers in the aggregate and periodically assess which competing markets, market makers, or electronic communications networks (ECNs) offer the most favorable terms of execution. Some of the factors a broker needs to consider when executing its customers' orders for best execution include: the opportunity to get a better price than what is currently quoted, the speed of execution, and the likelihood trade will be executed.
To learn more about the basics of trade execution including order routing, payment-for-order-flow, and internalization you should read Trade Execution: What Every Investor Should Know.
http://www.sec.gov/answers/bestex.htm