The Common Language Runtime
The CLR is the mechanism through which .NET code is executed. It is built upon a single, common language—IL—into which source languages are compiled and includes
mechanisms for executing the compiled code. This includes code verification and just-in-time(JIT) compilation, garbage collection and enforcement of security policies, and the provision of profiling and debugging services.

The CLR provides a lot of added value to the programs it supports. Because it controls how a .NET program executes and sits between the program and the operating system, it can implement security, versioning support, automatic memory management through garbage collection, and provide transparent access to system services. These features are explained in more detail in the section “How Does the .NET Architecture Work?”

It is an important fact that IL code doesn’t get executed itself, but is instead converted into platform native code before execution, a process known as JIT compilation. This compilation may happen at the time a program is installed or just before a piece of code runs, and it ensures that the code runs as efficiently as it can. The .NET Framework comes with several different JIT compilers, which are suited for different circumstances, such as whether you are primarily concerned with compilation speed or obtaining maximum optimization.