Add .NET Components article

docs/standard/components.md

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+---
+title: .NET Concepts
+description: Describes key .NET concepts such as the .NET Standard Library, .NET runtimes, tooling, and how they interplay.
+keywords: .NET, .NET Standard Library, .NET Standard, .NET Core, .NET Framework, Xamarin, MSBuild, C#, F#, VB, compilers
+author: cartermp
+manager: wpickett
+ms.date: 11/16/2016
+ms.topic: article
+ms.prod: .net
+ms.technology: .net-core-technologies
+ms.devlang: dotnet
+ms.assetid: 2e38e9d9-8284-46ee-a15f-199adc4f26f4
+---
+
+# .NET Components
+
+.NET is made up of a number of components.  It has a standard library, called the .NET Standard Library, which is a large set of APIs which runs everywhere.  This standard library is implemented by three .NET runtimes - .NET Framework, .NET Core, and Mono for Xamarin.  The .NET languages also run on any .NET runtime.  Additionally, there are tools on every platform which allow you to build projects.  These tools are the same regardless of your choice of runtime.
+
+Here's a graphical overview of each of the "pieces" of .NET and how they fit.
+
+![All .NET Concepts Together](media/dotnet-things.png)
+
+What follows is a brief explanation of each of the key concepts shown above.  
+
+## .NET Standard Library
+
+The .NET Standard Library is a set of APIs which are implemented by a .NET runtime.
+
+More formally, it is a formal specification of .NET APIs which defines a uniform set of APIs which enables two key each .NET runtime implements. and provides a set of contracts which enable portability across different .NET runtimes.
+
+The .NET Standard Library is also a base level for specifying dependencies for your code, where it is known as the .NET Standard.  You can "target" .NET Standard 1.0-1.6 right now, and will be able to target .NET Standard 2.0+ in the future.  If your code targets a version of the .NET Standard, it is guaranteed to run on any .NET runtime which implements that version.
+
+To learn more about the .NET Standard library and targeting the .NET Standard, check out [.NET Standard Library](library.md).
+
+## .NET runtimes
+
+There are three primary .NET runtimes which Microsoft actively develops and maintains: .NET Core, .NET Framework, and Mono for Xamarin.
+
+### .NET Core
+
+.NET Core is a cross-platform runtime optimized for server workloads.  It implements the .NET Standard Library, which means that any code that targets the .NET Standard can run on .NET Core.  It is the runtime used by ASP.NET Core and the Universal Windows Platform (UWP).  It is modern, efficient, and designed to handle server and cloud workloads at scale.
+
+To learn more about .NET Core, check out the [.NET Core Guide](../core/index.md).
+
+### .NET Framework
+
+.NET Framework is the historical .NET runtime that has existed since 2002.  It is the same .NET Framework existing .NET developers have always used.  It implements the .NET Standard Library, which means that any code that targets the .NET Standard can run on the .NET Framework.  It contains additional Windows-specific APIs, such as APIs for Windows desktop development with Windows Forms and WPF.  .NET Framework is optimized for building Windows desktop applications.
+
+To learn more about the .NET Framework, check out the [.NET Framework Guide](../framework/index.md).
+
+### Mono for Xamarin
+
+Mono is the runtime used by Xamarin apps.  It implements the .NET Standard Library, which means that any code that targets the .NET Standard can run on Xamarin apps.  It contains additional APIs for iOS, Android, Xamarin.Forms, and Xamarin.Mac.  It is optimized for building mobile applications on iOS and Android.
+
+To learn more about Mono, check out the [Mono documentation](http://www.mono-project.com/docs/).
+
+## .NET tooling and common infrastructure
+
+Tooling for .NET is also common across each implementation of .NET.  These include, but are not limited to:
+
+* The .NET languages and their compilers
+* Runtime components, such as the JIT and Garbage Collector
+* .NET project system (sometimes known as "csproj", "vbproj", or "fsproj")
+* MSBuild, the build engine used to build projects
+* NuGet, Microsoft's package manager for .NET
+* The .NET CLI, a cross-platform command-line interface for building .NET projects
+* Open Source build orchestration tools, such as CAKE and FAKE
+
+The main takeaway here should be that there is a vast amount of tooling and infrastructure which is common for any "flavor" of .NET you choose to build your apps with.
+
+## Next Steps
+
+Check out each of the below links to learn more about a particular topic:
+
+* [.NET Standard Library](library.md)
+* [.NET Core Guide](../core/index.md)
+* [.NET Framework Guide](../framework/index.md)
+* [C# Guide](../csharp/index.md)
+* [F# Guide](../csharp/index.md)
+* [VB.NET Guide](../csharp/index.md)