List of what classes are in what version of .Net (Win), Silverlight, Metro, and Mono (Mac OSX)

I'm an independent developer coming from Java to .Net Desktop. So I'm up against a learning curve and tend to think in terms of Java technologies.

Anyway I'm making a native desktop app for Windows (later you will buy it and we will all be happy!) The app will be made with C# and WPF for the best user experience (very important). While the priority is this best possible user experience I still want my code to be as portable as possible for future ports to other platforms. Obviously the GUI parts will be platform specific but there is also a lot of database type code that could be cross platform.

So for a certain package in the Full desktop API (I guess you guys call it the "framework") is there a place I can get all this compatibility info?

Each page of the MSDN documentation indicates which platforms and versions are supported. Just look at the Version Information section near the end of the page. For instance, it shows the following information for the Action delegate:

.NET Framework
Supported in: 4, 3.5
.NET Framework Client Profile
Supported in: 4, 3.5 SP1
Portable Class Library
Supported in: Portable Class Library

The information for Silverlight/Windows Phone is on a separate page, but if the type or member exists in Silverlight, you can access it from the Other versions dropdown at the top of the page.

Of course, it leaves Mono out, since Mono is not managed by Microsoft...

If you want to have maximum portability, you should probably have a look at the Portable Class Library

You're not going to find one site with all of this one it. Research is a giant part of development - you should be doing this on your own, rather than asking here. Google makes research easy - use it. Type "silverlight requirements" or "silverlight strengths vs weaknesses", etc.

You learn about technologies through research and use. There is no site out there that's just going to give you all the answers.