When you call your broker to buy or sell a stock or hit "enter" when placing an order through your online brokerage account that's only the beginning of the transaction. Your broker's firm must then send your order to a market to be filled. This process of filling your order is known as "trade execution."
Where and how your order is executed can impact the overall costs of the transaction, including the price you pay for the stock. To learn the basics of trade execution including order routing, payment-for-order-flow, and internalization you should read Trade Execution: What Every Investor Should Know.