The Microsoft .NET Framework 3 is the new managed code programming model for Windows®. It combines the power of the .NET Framework version 2.0 with new technologies for building applications that have visually compelling user experiences, seamless communication across technology boundaries, and the ability to support a wide range of business processes. These new technologies are Windows Presentation Foundation, Windows Communication Foundation, Windows Workflow Foundation, and Windows CardSpace.
The .NET Framework 3 is included as part of the Windows Vista operating system; you can install it or uninstall it using Windows Features Control Panel. This redistributable package is for Windows XP and Windows Server 2003.
The latest version (3.5) contains many new features building incrementally upon .NET Framework 2.0 and 3.0, and includes .NET Framework 2.0 service pack 1 and .NET Framework 3.0 service pack 1.