.NET Framework 3.5 is a software development framework created by Microsoft that allows developers to build and run applications on the Windows operating system. Released in 2007, it includes a large class library and supports various programming languages, making it easier to create desktop and web applications. This version introduced several new features, such as Language Integrated Query (LINQ) for data manipulation and ASP.NET AJAX for creating interactive web applications. It also improved existing libraries and tools, enhancing the overall development experience for programmers working with the .NET platform.