C# (pronounced see sharp) is a Object Oriented, Component Oriented and Most Advance Programming Language from Microsoft.
C# code is to be compiled before it Run. You can use C# to Create Windows Applications as well Web Applications. Software you see in General Life are Mostly powered by C#, because if Its Simplicity and Advance Features. There is nothing impossible to do with C#.
Now as I told earlier, C# source code must be compiled before it to Run. Even If you are creating Windows Application (Desktop based) or Web application (Browser based), you must compile the code.
You have following Software to to the Job of C# Compiler which are called IDE here and Compiling Task is called Implementation.
Main is Microsoft Visual C#. This is built in tool in Microsoft Visual Studio Express (Free).
Mono Project provides Open Source IDE for C# named MonoDevelop.
SharpDevelop is another Open Source Tool.