.NET framework is an open standard developed by Microsoft, its implementation is a set of libraries that contain functions to perform common tasks, so that programmers do not have to retype the repetitive parts of the program. In addition, .NET defines a standard way to communicate between two programs and programming languages called IL and C#.
Between implementations of .NET are Dot/GNU of the GNU project, Mono Framework of Novell (Owners of the distribution SUSE Linux) and .NET Framework from Microsoft. All three share the same basic structure, libraries and the same language equivalent. So that most programs to write for one of these implementations. NET will run in the other.
Despite being registered with an open standard ECMA and ISO, there are differences in implementations mainly because Microsoft has the lead in development and implementations Mono and Dot/GNU are lagging behind. On the other hand that Microsoft's implementation only works on Windows systems, while Dot/GNU focuses on systems derived from UNIX and Mono is intended to function on multiple platforms.
It is worth noting the similarity level architecture that .NET Mycrosistems and Sun Java have, with the difference that the latter is not an open standard. Also note that Microsoft and Mono have developed a tool for interactive applications, which competes with Adobe Flash. This tool is called SilverLight in the world of Microsoft and MoonLight in Mono.
Finally, .NET is the foundation of the Web development platform ASP.NET, direct competition from PHP and JSP. Likewise .NET is the framework for XBOX and XBOX 360 systems from Microsoft.