In a nutshell, the Common Language Runtime - the core classes/namespaces/etc. - are still 2.0. The difference is that you can now extend your work with Windows Presentation Foundation (Avalon), Windows Communication Foundation (Indigo), Workflow Foundation, and (I think) Cardspace.
If you are not familiar with the .NET Framework, you can find some general info here: [U]http://en.wikipedia.org/wiki/.NET_framework[/U]
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