By using the Visual Studio 2012 SDK, you can create tool windows, menu commands, editor extensions, isolated shell projects, and other extensions for Visual Studio.  The SDK also helps you test and debug your extensions in an experimental instance of Visual Studio so that your primary development environment is not changed unexpectedly.  After you install the SDK, you can use the shortcuts on the Start menu to find the online documentation, samples, and other developer resources.

Samples for building Visual Studio extension area available on the MSDN Code Gallery.