The .NET Framework provides a runtime environment called the
Common Language Runtime or CLR (similar to the Java Virtual
Machine or JVM in Java), which handles the execution of code
and provides useful services for the implementation of the
program. CLR takes care of code management at program
execution and provides various beneficial services such as
memory management, thread management, security management,
code verification, compilation, and other system services.
The managed code that targets CLR benefits from useful
features such as cross-language integration, cross-language
exception handling, versioning, enhanced security,
deployment support, and debugging.

CLR is the heart of the .Net framework.our application does 
not communicate os directly but go through CLR.It handles  
the execution of all .net applications.