.NET is a free, cross-platform and open source software development platform, created by Microsoft, made up of many tools, support many programming languages also provide libraries for developing many different type of software application.
Primarily .NET was meant to create applications, which would run on the Windows Platform, but now you can develop as well as execute application developed in .NET on Windows, Linux, macOS as well as Mobile Devices.
With .NET you can develop Desktop applications, Web applications, and MVC application, Services, WEB API, Mobile App, Gaming App and IoT App also.
.NET support to use multiple languages (C#, Visual Basic, F#), editors (text editor, visual studio, visual studio code, Sublime Text, VIM) and libraries
The first version of the .NET was released in the year 2002.The version was called .NET Framework 1.0 and current version of .NET Framework is 4.8 (released on 18-04-2019) and beside .NET Framework there are other implementation of .NET like .NET Core (Latest Version .NET Core 3.1 released on 03-12-2019) and Mono or Xamarin implementation.
.NET Framework is the original implementation of .NET. It supports running websites, services, desktop apps, and more on Windows.
.NET Core is a cross-platform implementation of .NET for running websites, services, and console apps on Windows, Linux, and macOS. It is open source on GitHub and maintained by Microsoft as well as .NET community.
Xamarin/Mono is a .NET implementation for running apps on all the major mobile operating systems, including iOS and Android. Mono is the original cross-platform implementation of .NET. It was started out as an open-source alternative to .NET Framework and transitioned to targeting mobile devices as iOS and Android devices became popular. It can be thought of as a community clone of the .NET Framework.
.NET Standards is a formal specification of the APIs that are common across all these .NET implementations. It allows libraries to build against the agreed on set of common APIs, ensuring they can be used in any .NET application—mobile, desktop, IoT, web, or anywhere you write .NET code.
Sukh Vir Singh's Blog 