Microsoft .Net Assembly is a logical unit of code, it contains code that the Common Language Runtime (CLR) executes. Assembly is really a collection of types and resource information that are built to work together and form a logical unit of functionality. During the compile time Metadata is created, with Microsoft Intermediate Language (MSIL), and stored in a file called a Manifest . Both Metadata and Microsoft Intermediate Language (MSIL) together wrapped in a Portable Executable (PE) file. Manifest contains information about itself. This information is called Assembly Manifest, it contains information about the members, types, references and all the other data that the runtime needs for execution.

Every Assembly you create contains one or more program files and a Manifest. There are two types program files : Process Assemblies (EXE) and Library Assemblies (DLL). Each Assembly can have only one entry point (that is, DllMain, WinMain, or Main). We can create two types of Assembly, private Assembly and shared Assembly . A private Assembly is used only by a single application, and usually it is stored in that application's install directory. A shared Assembly is one that can be referenced by more than one application. If multiple applications need to access an Assembly, we should add the Assembly to the Global Assembly Cache (GAC).