The CLI (Common Language Infrastructure) is the definition of the fundamentals of the .NET framework – the Common Type System (CTS), metadata, the Virtual Execution Environment (VES) and its use of intermediate language (IL), and the support of multiple programming languages via the Common Language Specification (CLS).
The CLR (Common Language Runtime) is Microsoft’s primary implementation of the CLI. Microsoft also have a shared source implementation known as ROTOR, for educational purposes, as well as the .NET Compact Framework for mobile devices.

Non-Microsoft CLI implementations include Mono and DotGNU Portable .NET.