.NET is a general purpose software framework originally released by Microsoft for the development of programs in environments such as desktops, servers, mobile devices such as phones and tablets and constrained embedded systems. This means that the .NET framework can be used to create cross-platform native and web applications. It was originally released in the year 2000.
The .NET framework consists of a large class library called Framewok Class Library (FCL) and provides language interoperability across several programming languages such as C#, VisualBasic.NET, F#, Python, among others. Programs written wich such library execute in a software environment named Common Language Runtime (CLR), which is a virtual machine that provides application services like memory management, exception handling and security. Application code that takes advantage of such services is said to be managed code. The framework also allows interoperability with unmanaged code, which means that managed code can run side by side with code outside of the CLR itself, for example to use legacy programs.
The current stable version of the .NET framework is 4.5.2 which was released on May 5, 2014, and at the time of this writing the next .NET Framework 4.6 is in preview.
.NET 2015
The next generation of .NET technologies is being marketed with the term .NET 2015 and it includes important updates to the framework, the runtimes, compilers, libraries, release licenses, applications models and development workflows of applications and of the technologies. In short, Microsoft aims to significantly improve the developer’s experience by open sourcing the full server-side .NET stack as well as other important development elements, making the framework officially cross-platform by adding support to the Linux and Mac OS platforms.
Regarding the software framework itself, from now on there will be 2 separate .NETs
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.NET Framework 4.6 will be the next version of the full framework, building upon 4.5.2. This is the familiar .NET most people already know, which will be included in Windows Vista and above. The .NET Framework 4.6 will not be open sourced.
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.NET Core 5 is a general purpose, modern and open source version of the framework currently in development, which when released will be supported by Microsoft and the community on Windows, Linux and Mac OS X. It is the foundation of all future .NET platforms and it’s released under the MIT license.
It is modular by design, and since it can be deployed itself as a Nuget package, it will make it possible for applications to ship with its own .NET framework without affecting other .NET applications running on the same machine, greatly simplifying deployment and package management. This means that individual applications can exist isolated and will no longer require machine-wide updates to the .NET framework.
The following components are part of .NET 2015 too and are open source already:
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New versions of the .NET libraries ASP.NET Web Forms and MVC 5
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.NET Compiler Platform, AKA Roslyn. It provides C# and Visual Basic compilers with rich code analysis APIs.
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RyuJIT, the new default just-in-time (JIT) compiler for .NET on x64 architectures.
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ASP.NET 5, the new application model for building modern and lean web applications.
The following components are part of .NET 2015 too but are not open source as of now:
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Windows Forms
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WPF
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.NET Native
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Universal Windows Apps, AKA Universal Windows Platform
Involvement with organizations
The .NET Foundation is an independent organization with the goal of improving open source development and community collaboration around the .NET Framework and related technologies. It was founded by Microsoft in April 2014. Miguel de Icaza, co-founder and CTO of Xamarin is on the board of directors.
Microsoft continues to be involved with the Mono project, Xamarin and Unity.
Mono is an open source project that develops a Ecma-standard compliant, .NET Framework-compatible set of tools, most notably a C# compiler and a CLR.
Xamarin is a software company that creates .NET development tools to create shared-code native applications on iOS, Android and Windows Phone.
Unity Technologies is the organization that develops Unity, a cross-platform game engine used to develop games for consoles, PC, mobile devices and websites. The engine’s scripting is built on Mono.