What is "clr"

Common Language Runtime (CLR) is the component of Microsoft’s NET virtual machine. It is responsible for managing the execution of NET programs. In a process known as just-in-time compilation, compiled code is converted into machine instructions that, in turn, are executed by the computer’s CPU. CLR provides additional services, including memory management, security type, and exception handling. All programs written for NET., regardless of the programming language, are executed by CLR. It provides exception handling, garbage collection and thread management. CLR is common to all versions of the NET framework.

source: http://en.wikipedia.org/wiki/Common_Language_Runtime