what is the common language runtime (clr)?

Answer 1

"The Common Language Runtime (CLR) is the virtual machine component of Microsoft's .NET initiative. It is Microsoft's implementation of the Common Language Infrastructure (CLI) standard, which defines an execution environment for program code. The CLR runs a form of bytecode called the Microsoft Intermediate Language (MSIL), Microsoft's implementation of the Common Intermediate Language...."

Answer 2

As the name suggests, it's the common interpreter to the .NET languages (VB.NET, C#, C++ etc). It allows all of those languages to share a common set of commands (several tens of thousands) to perform certain functions.