C# is a primarily object-oriented programming language originally created by Microsoft. It is now an ECMA-approved standard and is not a proprietary language. It is built upon the .NET platform, which gives it a great amount of power for its relative simplicity. It is strongly-typed and contains a garbage collector to automatically release unreferenced memory. It is also particularly useful as a scripting platform since it is possible to access the C# compiler from within the code itself.
C# can be run by either the proprietary .NET runtime or the open-source Mono runtime. The former only works on Microsoft systems (Windows, Xbox, Windows Phone), but the latter "runs on Linux, Microsoft Windows, Mac OS X, BSD, and Sun Solaris, Nintendo Wii, Sony PlayStation 3, Apple iPhone. It also runs on x86, x86-64, IA64, PowerPC, SPARC (32), ARM, Alpha, s390, s390x (32 and 64 bits) and more. Developing your application with Mono allows you to run on nearly any computer in existance[sic]". However, Mono is not as fast as the .NET runtime, but that is a small price to pay for supporting so many more platforms.