The Common Language Runtime (CLR) is a virtual machine environment and part of the .NET Framework. It contains:

• A portable bytecode language called Common Intermediate Language (abbreviated CIL, or IL)
• A Just-In-Time compiler that generates machine code
• A tracing garbage collector that provides automatic memory management
• Support for lightweight sub-processes called AppDomains
• Security mechanisms through the concepts of verifiable code and trust levels

Code that runs in the CLR is referred to as managed code to distinguish it from code running outside the CLR (usually native code) which is referred to as unmanaged code. There are various mechanisms that facilitate interoperability between managed and unmanaged code.