ASP.NET Core is a free, open-source web framework developed by Microsoft. It provides features that enable building the backend for modern web applications, as well as web APIs. The programming language that is used for the development of ASP.NET Core is C# or any other .NET-based programming language.

ASP.NET Core is a redesign of the popular ASP.NET Model View Controller (MVC) and ASP.NET Web API frameworks. The result is a leaner and more modular framework that can run on the full .NET Framework on Windows and .NET Core on other platforms.

Both parts of the framework, MVC and Web API, help in creating modern web applications. MVC is for building traditional web applications in which rendering is done on the server-side, but also supports integration with modern JS libraries and client-side rendering. ASP.NET offers many web development features out of the box, such as security, data validation, deployment, and more. ASP.NET Web API is for creating RESTful web services that serve modern frontend applications, mobile apps, and any other endpoints.

ASP.NET Core is a popular choice but is definitely not alone in the world of backend development. It is similar to other frameworks such as Laravel (PHP), Spring (Java), Ruby on Rails (Ruby), Django (Python), and others. Each has its own advantages and disadvantages. I chose ASP.NET Core for this book as it is one of the top frameworks out there, runs on the powerful C# language, and has wonderful IDE support.