PE File.

Figure 1 - PE File

• The CLR header stores information to indicate that the PE file is a .NET executable and the CLR data section contains metadata and Microsoft Intermediate Language (MSIL, or IL for short) code.
• Every common language runtime–compliant development tool compiles its own source code into IL code. Because all development tools produce the same IL, regardless of the language in which their source code is written, differences in implementation are gone by the time they reach the common language runtime.
• Metadata is data that is used to describe classes and what they can do, separate from the code of the class itself. It is important to understand that metadata is not part of the class in the same way that variables and methods are, but instead it is used to describe classes.The CLR uses metadata for many purposes including; locating and loading classes, laying out objects in memory, finding out what methods and properties a class has, enforcing security and discovering the class’s transactional behaviour. You can ask an object at runtime for this type of information such as its type, methods, properties, events and so on.Most of the metadata associated with a class is provided by the compilation process, but it is possible to create your own metadata items, called attributes, and attach them to your own classes.
• IL code : Code the compiler produced as it compiled the source code. At runtime, the CLR compiles the IL into native CPU instructions.

　　Ref: Programming Design Pattern and CLR Via C#.