The .NET Framework is a development and execution environment that allows different programming languages & libraries to work together seamlessly to create applications that are easier to build, manage, deploy, and integrate with other networked systems.

The .NET Framework provides the basic infrastructure that applications need to make Microsoft's .NET vision of connecting information, people, systems, and devices a reality:  

Support for standard networking protocols & specifications
The .NET Framework uses standard Internet protocols and specifications like TCP/IP, SOAP, XML, & HTTP to allow a broad range of information, people, systems, and devices to be connected

Support for different programming languages
The .NET Framework supports a variety of different programming languages so developers can pick the language of their choice

Support for programming libraries developed in different languages
The .NET Framework provides a consistent programming model for using prepackaged units of functionality (libraries) which makes application development faster, easier & cheaper

Support for different platforms
The .NET Framework is available for a variety of Windows platforms, which allows people, systems, and devices to be connected using different computing platforms. E.g. People using desktop platforms like Windows XP or device platforms like Windows CE can connect to server systems using Windows Server 2003.