What is it?
C# is a high level object-oriented and strongly typed programming language created by Microsoft.
It would be used primarily in creating enterprise-grade web applications, desktop applications and games.
What is it like?
The language itself is quite neat and easy to follow, with plenty of good documentation available to bring oneself up to speed.
When used correctly, performance is reasonably good but it is not designed to compete with the likes of C and C++.
I used it in conjuction with ASP.NET MVC framework when developing the server side component for the iBox TV guide.