C# is a Microsoft language standard. Microsoft provides its own "Introduction and Overview" here [L1 ] (but note that MS moves its Web pages around quite capriciously; this page is likely to expire at any time).
It was introduced with the .NET Framework.
Visual Studio .NET ships with a C# compiler.
Mono is an Open Source version of C# and its related libraries.
[explain how it's like Java and C++] [who was the main designer of C#?] [what is it used for?] [can it be used with Tcl or Tk?]
2003/01/06 JJM - Yes, it can be used with Tcl/Tk via TCLBridge.
There's a great deal of competition for mindshare among those who think they want to program with C#. Among the leading online resources are: