The size of the documentation corpus on .NET Framework is pretty large. Below, you can find the pointers to resources for various pieces of the .NET ecosystem, from the platform to various application stacks that are implemented and run on top of it.
MSDN is always a very good first stop for any documentation needs you may have. You can visit the MSDN CLR page to start exploring the .NET Framework.
Currently, we are building out a new set of documents that is covering the .NET Core scenarios and features. You can take a look at the current set of finished articles on .NET Core ReadTheDocs project. As we are working towards building out this documentation corpus, there will be more additions and fixes to this set of documents. Of course, it is all open source, so you can also contribute to the documentation on GitHub.
You can also visit our GitHub repo documentation and get introduced to the project. I would suggest reading through the excellent “Book of the Runtime”, especially if you are interested in the more low-level technical details.
.NET Version Manager (DNVM) and .NET Execution Environment (DNX) are very important parts of the new .NET ecosystem, especially if you wish to work with .NET Core. The documentation on these components can help you understand the details and how they are used, especially the overview document.
ASP.NET team has created brand new content for ASP.NET 5 at docs.asp.net. These documents contain the details about ASP.NET 5 release in general, and its various pieces in particular, such as ASP.NET MVC6.
“Roslyn” is the new managed compiler for C# and Visual Basic languages that is part of Visual Studio 2015. The best starting point for learning about “Roslyn” is to consume the overiew document. After that, you can try the samples & walkthroughs.