Common Language Specification

Common Language Specification (CLS) is a set of basic language features that .Net Languages needed to develop Applications and Services , which are compatible with the .Net Framework. When there is a situation to communicate Objects written in different .Net Complaint languages , those objects must expose the features that are common to all the languages .

Common Language Specification (CLS) ensures complete interoperability among applications, regardless of the language used to create the application.

Common Type System

In order that two language communicate smoothly CLR has CTS (Common Type
System).
Example in VB you have “Integer” and in C++ you have “long” these datatypes are not
compatible so the interfacing between them is very complicated. In order that these two different languages communicate Microsoft introduced Common Type System. So “Integer” data type in VB6 and “int” data type in C++ will convert it to System.int32, which is data type of CTS.
CLS is subset of CTS.

Common Language Runtime

The Common Language Runtime is the runtime in the .Net environment that compiles the source code in to an intermediate language. This intermediate language is called the Microsoft Intermediate Language(MSIL).
During the execution of the program this MSIL is converted to the native code or the machine code. This conversion is possible through the Just-In-Time compiler. During compilation the end result is a Portable Executable file (PE).
This portable executable file contains the MSIL and additional information called the metadata. This metadata describes the assembly that is created. Class names, methods, signature and other dependency information are available in the metadata. Since the CLR compiles the source code to an intermediate language, it is possible to write the code in any language of your choice. This is a major advantage of using the .Net framework.
The other advantage is that the programmers need not worry about managing the memory themselves in the code. Instead the CLR will take care of that through a process called Garbage collection(GC). This frees the programmer to concentrate on the logic of the application instead of worrying about memory handling.