CLR and CLI - What is the difference?

The CLR (Common Language Runtime) is Microsoft's implementation of the VES (Virtual Execution System). The VES along with the CTS (Common Type System), the CLS (common language specification) and the metadata specification are all part of the CLI (Common Language Infrastructure) specification.

The VES is a standardized virtual machine specification that must be implemented in order to load and execute CIL (Common Intermediate Language) modules (dll and exe). A VES implementation also provide runtime services such as garbage collection and security.

ECMA C# and Common Language Infrastructure Standards

The CLR is Microsoft's implementation of the CLI standard.

CLR is the execution environment in which a .NET application is safely hosted/run.

You can see it as .NET's private Operating System that initiates and loads just before a .NET application starts.

The CLR takes care of certain essential requirements of any .NET application that otherwise would require lot of deliberate code to be written in order to implement; requirements that are holistic in nature and essential to any kind of application to run in a good, efficient and safe manner [e.g. Handle memory allocation and release it when not required, avoid dangling pointers, avoid type-casting errors etc. ]

CLI on the other hand is a specification/set of guidelines that explains how to implement an application execution environment and the nature of generated application code that allows for multiple high-level languages to be used on different computer platforms without being rewritten for specific architectures.

CLI is developed by Microsoft and standardized by ISO and ECMA.

The CLR is a practical implementation of CLI's VES [Virtual Execution System] section and forms one of the core components of the MS.NET platform

In a layman's language, CLI is a recipe while CLR is the cuisine :-)