CSharp (actually C♯, also often called C#) is a modern programming language that has first been released in 2001. It is standardized, type-safe, and object-oriented. It was developed on behalf of Microsoft and was also initially used almost exclusively for Windows.
Today, C# is also used for other operating systems. Nevertheless, it is more often found in applications that are only intended to run on Windows. Especially desktop applications are often programmed in C#.