.NET Framework is Microsoft's platform for application development that provides a unified programming model for building various applications. It includes the Common Language Runtime (CLR) for executing managed code, the Framework Class Library (FCL) with core functionality like ASP.NET and Windows Forms, and support for multiple programming languages. The CLR handles memory management, security, and code execution while the FCL contains reusable types for common tasks. This allows developers to create applications using languages like C# and VB.NET that run across Windows and web platforms.