The Microsoft .NET Framework 3.5 is a software development platform released in 2007 that provides tools and libraries for building and running applications on Windows. It includes features like Language Integrated Query (LINQ) for data manipulation and Windows Communication Foundation (WCF) for building networked applications, enhancing the capabilities of developers. This version builds upon previous releases, adding support for ASP.NET AJAX to create more interactive web applications. It is designed to improve performance and security, making it easier for developers to create robust applications that can run on various Windows operating systems.