I found explanations of CLR and CLI in the online MSDN Library Visual Studio / Overview of .NET Framework section. Click on the links to get more detail.
Common Language Runtime (CLR)
The .NET Framework provides a run-time environment called the common language runtime, which runs the code and provides services that make the development process easier.
Cross-Language Interoperability
Describes the common language runtime's built-in support for language interoperability. Language interoperability is the ability of code to interact with code that is written by using a different programming language. Language interoperability can help maximize code reuse and improve the efficiency of the development process.