The .NET Framework is a set of software tools that provides everything you need to run programs developed using Microsoft's .NET technology. This includes runtime engines, memory management tools, support for ASP.NET applications, and a whole lot more.
Updates to the Windows Presentation Foundation (WPF) include extended high DPI support, more consistent and reliable touch handling, and transparent child window support.
Changes in ASP.NET 4.6 include authentication updates and support for the super-efficient HTTP/2.
A smarter Garbage Collector should improve memory utilisation and reduce latency, without requiring any code changes.
A more flexible Windows Communication Foundation now supports SSL version TLS 1.1 and TLS 1.2, in addition to SSL 3.0 and TLS 1.0, when using NetTcp with transport security and client authentication.
The list goes on, with the new RyuJIT JIT compiler, SIMD support, and of course a stack of language extensions and improvements for C# 6, F# 4, and VB 14.