.NET is a software development framework created by Microsoft that allows developers to build and run applications on various platforms. It provides a large library of pre-built code, known as the Base Class Library, which simplifies common programming tasks. Developers can use multiple programming languages, such as C# and VB.NET, to create applications for web, desktop, and mobile devices. The framework supports different types of applications, including web services, cloud-based applications, and desktop software. With the introduction of ASP.NET, developers can create dynamic websites and web applications efficiently. Overall, .NET aims to enhance productivity and streamline the development process.