.NET Runtime

Also known as the Common Language Runtime.
The CLR (Common Language Runtime) is a run-time environment that is provided by the .NET Framework.
This runtime manages the life cycle and executes .NET applications.
Programming languages that target the CLR created "managed code".
The CLR provides a common type system to allow the integration of different programming languages (VB.NET, C#, F#, C++/CLI)
When any managed code is compiled it is converted into an Intermediate Language (or Common Intermediate Language).
Code that executes outside the control of the CLR is unmanaged code.
The CIL is then compiled at run-time by the CLR.

This intermediate language means that code written in one language (eg C#) can easily call code written in a different language (VB.NET).
This allows programmers to write code in their favourite language and for that code to be easily integrated with code that has been written in a different language.

The CLR uses something calls Common Type System (CTS) to strictly enforce type-safety. This ensures that all classes are compatible with each other, be describing types in a common way. CTS defines how types work within the runtime (their declaration and usage), which enables types in one language to interoperate with types in another language, inclugin cross-language exception handling
The CLR is a run-time execution engine that is responsible for executing code within the .NET environment, providing services such as security, memory management and remoting.
The Common Language Specification (CLR) is a set of rules and constraints that compiler and library writers need to follow to be sure that the languages and code they produce will interoperate with other .NET langauges
.NET resources are handled differently from Win32

Intermediate Langauge (IL)

Also known as the Common Internediate Language (CIL).
All .NET langauges compile down into an intermediate form called Intermediate Language (IL). IL can be thought of as a type of object-oriented assembler langauge.
It allows you to do cross-langauge object-oriented programming easily calling member in Managed C++ classes from Visual Basic and vice versa and even inheriting from a Managed C++ class in Visual Basic