what is Visual Studio 2010 Shell?

Visual Studio Shell is a base for developing some parts of IDEs. If you want, you can use it to create your own programming environment instead of starting from the zero.

All Visual Studio (C#, C++, VB.Net, Web), shares some of the shell functions, implementing the environment specific on each one, and configuring bunch of parameters on the shell.

Some 3rd party uses Visual Studio Shell too, like Atmel Studio.

More about: http://msdn.microsoft.com/en-us/library/vstudio/bb685612.aspx

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^{Note: I don't think you will open any project in the shell itself, it will open under the correct implemented IDE troght a selector application and its registered IDEs.}