On disk, assemblies are Portable Executable (PE) files. PE files are not new. The format of a .NET PE file is exactly the same as a normal Windows PE file. PE files are implemented as DLLs or EXEs. Logically (as opposed to physically), assemblies consist of one or more modules. Note, however, that an assembly must have exactly one entry point: DLLMain, WinMain, or Main. DLLMain is the entry point for DLLs, WinMain is the entry point for Windows applications, and Main is the entry point for DOS and Console applications.
Modules are created as DLLs and are the constituent pieces of assemblies. Standing alone, modules cannot be executed; they must be combined into assemblies to be useful.
Deploy and reuse the entire contents of an assembly as a unit. Assemblies are loaded on demand and will not be loaded if not needed.