The language the Common Language Runtime is written in or compiled into. Unfortunately as there are many different Runtimes/Interpreters for IL Code, I can not tell you any more precise without you telling me wich specific one we are talking about.
.NET Programms are compiled into the Intermediate Language. The IL is then run by the CLR. If you think that sounds like Java Bytecode being executed by the Java Application - you are damn right! It was most definitely an inspiration. It is a step up too, however.
For several points of view, .NET Programms are as interpreted as a Batch File, Java ByteCode, JavaScript or PHP/Python script. There are some things all of those can do, that they will not make available for Programms running in them.