.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 languages like C#, VB.NET, and F# to create applications for web, desktop, and mobile devices. The framework supports multiple operating systems through .NET Core, which is a cross-platform version of .NET. This means developers can write code once and run it on different systems, such as Windows, macOS, and Linux. Overall, .NET enhances productivity and streamlines the development process.