C# is a general-purpose, cross-platform programming language from Microsoft and the main language of .NET, a free, cross-platform, open-source development platform. It is object-oriented, which means that programs are built from classes that combine data with the code that works on it, and it also takes many features from functional programming.
C# is a compiled language: the dotnet build command compiles the source files into a binary package, and dotnet run runs it. Its syntax belongs to the C family, close to C, C++, Java and JavaScript. Language Integrated Query (LINQ) gives one query syntax for in-memory collections, XML, JSON and databases, and the async and await keywords let asynchronous code, such as a web request, read like sequential code. With .NET, C# builds web backends with ASP.NET Core, cloud services on Azure, desktop and mobile apps with .NET MAUI, and games with Unity.
The language and the .NET platform are free to use. Microsoft documents them on Microsoft Learn, with a reference and an official language specification.