Suppose you decide to purchase an airline ticket online through a travel Web site. You search the Web site's database for an available flight. Once you select an itinerary, the Web application communicates with the airline's database to book the flight and secure a confirmation. If you receive a message that the flight is no longer available, it is usually because of the difference between the travel Web site's database and the real-time availability of flights in the airline's database.

If the airline reservation system used a Web Service as a method of communication, the travel Web site could simply send input (such as your name, the date, time of flights, and so on) to the Web Service. The Web Service would then pass the airline's output (flight confirmation, price, and ticket number) back to the travel Web site.