Not all web applications return an HTML view as a response to an HTTP request. Sometimes, a client only wants some data from your application, and it wants to handle how that data will be formatted.

For EXAMPLE, let's say your application supports both web and mobile interfaces. Instead of WRITING two separate projects which return HTML and mobile views, you can write a single application that only RETURNS the specific data that the clients need. Once the clients receive this data, they format it accordingly. The web client renders the HTML using view templates and JavaScript, and the mobile clients generate the appropriate mobile view for its specific platform.

An application might also need to communicate with another application to fetch the data that it needs. For example, when you go to Amazon.com, it communicates with hundreds of other services and applications to retrieve data and renders the final HTML page you see.

Such back-end applications, which provide data, are commonly known as RESTful web services. REST protocol uses verbs LIKE GET, POST, PUT, DELETE to communicate between multiple applications. The client and server can be written in different languages and technologies and STILL work together without knowing about each other, as long as each side knows the format of the data that is getting sent.

ASP.NET Core supports creating RESTful services, also known as web APIs, using C#. A Web API consists of one or more controllers that derive from ControllerBase class.

[PostController]
[Route("[controller]")]
public class PostController : ControllerBase

An MVC controller derives from the Controller class. However, a Web API controller should derive from the ControllerBase class. The reason is that Controller derives from ControllerBase and provides additional support for views, which you don’t need for web API requests.

That said, you can use a controller for both rendering views and data. That is, it can act as both an MVC controller and a Web API controller. In this case, you can derive the controller from the Controller class.