.NET is a software framework and platform developed by Microsoft that runs primarily on Microsoft Windows. It includes a large class library called Framework Class Library (FCL) and provides language interoperability across several programming languages. Some key aspects include the Common Language Runtime (CLR) which handles memory management, exception handling and security, and supports languages like C#, Visual Basic .NET and F#. The .NET Framework also includes APIs for building Windows-based applications, web applications and web services.