When I was first getting started on the transition from Classic ASP to ASP.NET, one of the most daunting tasks was learning the .NET Framework. Learning C# was comparatively easy, because a series of courses introduced the language gradually. But for learning the .NET Framework, the enterprise architects at the company I worked for told me what I should do is "read through the class library to find out what's available."
There are tens of thousands of classes in the .NET API class library. Who has time to read all that? And yet they had a point about the value of being familiar with what's available.
To make this task more manageable, the ASP.NET documentation for .NET 4 provides an annotated list of web-related namespaces. The list optimizes your time by omitting namespaces that have classes that are deprecated or otherwise less useful to most developers. For each namespace there's a brief explanation of its purpose and what it contains. You can quickly get an overview of what's available and drill down into particular classes for areas of special interest to you.
To see this new overview, go to the NET API Reference section of the ASP.NET Overview.
For the entire .NET Framework, there's no comparable list of selected namespaces, but you can peruse the complete list at the .NET Framework Class Library.