Calling web services

Suppose we need to book a flight ticket online. We can do this by using either the website of the flight's brand (such as Lufthansa, Emirates, and so on), or a travel booking website (such as ClearTrip, MakeMyTrip, and so on). How is it that we can do the same task from two or more different websites?

The website of the flight's brand provides some APIs with which the travel booking websites work. These API can be freely available or charged by a contract, which is for the provider and the other third-party involved to decide. These APIs are also called web services.

A web service is more or less a method that is called over the Internet. Only the provider is fully aware of the internal working of these sites. Those who use the web service are only aware of the purpose and its possible outcome.

Many applications require/prefer to use third-party APIs to complete common tasks for various reasons, such as common norms in the business domain, easier means to provide...