I am a dotnet developer that super easy to forget technical abbreviations.

And here i found out a short and clear explanation on what is CIL and CLR, Wikipedia!

The Common Language Runtime (CLR) is a special run time environment that provides the underlying infrastructure for Microsoft’s .NET framework. This runtime is where the source code of an application is compiled into an intermediate language called CIL , originally known as MSIL (Microsoft Intermediate Language). When the program is then run, the CIL code is translated into the native code of the operating system using a just-in-time (JIT) compiler.

This intermediate language is used to keep the environment platform-neutral and as a result, supports all .NET languages such as C# or VB.NET.

This image from Mark Raju explain CIL, CLR & JIT pretty well: