C# is a modern object-oriented, general-purpose programming language, created and developed by Microsoft together with the .NET platform. There is highly diverse software developed with C# and on the .NET platform: office applications, web applications, websites, desktop applications, mobile applications, games, and many others.

 C# is a high-level language that is similar to Java and C++ and, to some extent, languages like Delphi, VB.NET and C. All C# programs are object-oriented. They consist of a set of definitions in classes that contain methods and the methods contain the program logic – the instructions which the computer executes. 

Nowadays C# is one of the most popular programming languages. It is used by millions of developers worldwide. Because C# is developed by Microsoft as part of their modern platform for the development and execution of applications, the .NET Framework, the language is widely spread among Microsoft-oriented companies, organizations, and individual developers. For better or for worse, as of this book writing, the C# language and the .NET platform is maintained and managed entirely by Microsoft and is not open to third parties. Because of this, all other large software corporations like IBM, Oracle and SAP base their solutions on the Java platform and use Java as their primary language for developing their own software products.

 Unlike C# and the .NET Framework, the Java language and platform are open-source projects that an entire community of software companies, organizations, and individual developers take part in. The standards, the specifications and all the new features in the world of Java are developed by workgroups formed out of the entire Java community, rather than a single company (as the case of C# and .NET Framework). 

The C# language is distributed together with a special environment on which it is executed, called the Common Language Runtime (CLR). This environment is part of the platform .NET Framework, which includes CLR, a bundle of standard libraries providing basic functionality, compilers, debuggers, and other development tools. Thanks to the framework CLR programs are portable and, once written they can function with little or no changes on various hardware platforms and operating systems. C# programs are most commonly run on MS Windows, but the .NET Framework and CLR also support mobile phones and other portable devices based on Windows Mobile, Windows Phone and Windows 8. C# programs can still be run under Linux, FreeBSD, iOS, Android, macOS X, and other operating systems through the free .NET Framework implementation Mono, which, however, is not officially supported by Microsoft.