CLR

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.

The Common Language Runtime is the underpinning of the .NET Framework. 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.

Common Type System (CTS) describes how types are declared, used and managed in the runtime and facilitates cross-language integration, type safety, and high performance code execution.

The Common Language Specification (CLS) is an agreement among language designers and class library designers to use a common subset of basic language features that all languages have to follow.

Reference: http://www.arunmicrosystems.netfirms.com/clr.html