"If you create software development tools, you’ll want to consider building on the Visual Studio 2008 Shell. A streamlined Visual Studio development environment, the Visual Studio Shell provides the core foundation so you can focus on building your application’s unique features. Flexible customization options help you deliver optimized experiences for specific markets."
What I find most interesting is the two modes - integrated and isolated.
In integrated mode applications built on the integrated Shell will automatically merge with any other editions of Visual Studio installed on the same machine. e.g. Iron Python
In isolated mode applications built with the isolated Shell will run side-by-side with any other editions of Visual Studio installed on the same machine.