From Microsoft:Microsoft NET Framework 3.5 builds incrementally on the new features added in NET Framework 3. For example, Windows Workflow Foundation (WF), Windows Communication Foundation (WCF), Windows Presentation Foundation (WPF), and Windows CardSpace. The features have been added as new assemblies to avoid breaking changes. It includes deep integration of Language Integrated Query (LINQ) and data awareness, ASP.NET AJAX lets you create more efficient, more interactive, and highly-personalized Web experiences that work across all the browsers, new Web protocol support for building WCF services including AJAX, JSON, REST, POX, RSS, ATOM, and several new WS-* standards, and new classes in .NET Framework 3.5 base class library (BCL) that address many common customer requests.