Common Intermediate Language (formerly known as MSIL, or Microsoft Intermediate Language) is a low-level assembly-type programming language using the .NET-platform. It is the language into which high-level .NET-languages such as C#, Visual Basic.NET, J# and F# are compiled. CIL is then made into an executable (.exe) which can then be run.
CIL is very complex compared to, say, C#, and a single instruction in C#, like:
... could be written in CIL as:
ldstr "Hello, World!"
call void mscorlib
People writing their applications in CIL directly will usually get the benefits of more powerful (speedier) applications that take up less hard disk space. However, if time is of the essence, one should consider using another .NET-language, such as C#.