What is an API? API is nothing but an application program interface. As we already said the name itself says it is an interface. How does API Testing work? It is all about requests and responses between the client and server. You want something, you request something so there is a way it gets carried to the server and you get the response back from the server. In other words, it’s all about your requesting and you know getting some responses from the provider. Example: Now open UBER and see. Once the site is loaded, it gives you an ability to log in or continue with Facebook and Google. In this case, Google and Facebook are also exposing some level of users’ information. There is an agreement between UBER and Google/Facebook that has already happened. That’s the reason it is letting you sign up with Google/ Facebook.