C# is a modern, object-oriented programming language developed by Microsoft, primarily used for building a wide range of applications, from web and desktop applications to mobile and game development. It is a statically typed, type-safe language that supports advanced features like asynchronous programming (with async and await), LINQ (Language Integrated Query) for data manipulation, and lambda expressions for functional programming. .NET, the framework that C# runs on, provides a comprehensive environment for building and running applications, with key components like the Common Language Runtime (CLR) for executing code, and the Base Class Library (BCL) for common functionality like file I/O, networking, and data access.