DotNet is a software development framework created by Microsoft that allows developers to build and run applications on various platforms. It supports multiple programming languages, including C#, VB.NET, and F#, making it versatile for different types of projects, from web applications to desktop software. The framework provides a large library of pre-built code, known as the Base Class Library, which simplifies common programming tasks. Additionally, DotNet has evolved into DotNet Core, a cross-platform version that enables developers to create applications that can run on Windows, macOS, and Linux.