Assemblies

An assembly is the functional unit of sharing and reuse in the Common Language Runtime. It is the equivalent of JAR (Java Archive) files of Java.
Assembly is a collection of physical files package in a .CAB format or newly introduced .MSI file format. The assemblies contained in a .CAB or .MSI files are called static assemblies, they include .NET Framework types (interfaces and classes) as well as resources for the assembly (bitmaps, JPEG files, resource files, etc.). They also include metadata that eliminates the need of IDL file descriptors, which were required for describing  COM components.
The Common Language Runtime also provide API's that script engines use to create dynamic assemblies when executing scripts. These assemblies are run directly and are never saved to disk.
Microsoft has greatly  diminished the role of Windows Registry system with introduction of assemblies concept, which is an adaptation of Java's JAR deployment technology.
Assemblies is an adaptation, but not a copy of Java's JAR technology. It has been improved upon in some ways, for example it has introduced a versioning system. However, since the .NET framework is skewed towards the Windows architecture some of the Java's JAR portability features may have been sacrificed.
Again, similar to JAR files, the assemblies too contain an entity called manifest. However, manifest in .NET framework plays somewhat wider role. Manifest is a metadata describing the inter-relationship between the entities contained in the assemblies like managed code, images and multimedia resources. Manifest also specifies versioning information.
The manifest is basically a deployment descriptor, having XML syntax. Java programmers can relate it with J2EE (Java 2 Enterprise Edition) deployment descriptors for EjB (Enterprise Java Beans) applications.
The Microsoft documentation stress that assemblies are \"logical dlls\". This may be a reasonable paradigm for VB or C++ programmers, but  Java programmers will find it easier, if we visualize assemblies as an extension of JAR concept. However, unlike JAR, each assembly can have only one entry point defined, which can be either DllMain, WinMain, or Main.
As stated earlier, Assemblies have a manifest metadata. This contains version and digitally signed information. This purports to implement version control and authentication of the software developer. Version and authentication procedure is carried out by the runtime during loading the assembly into the code execution area.
Again, much like Java's trusted lib. concept, .NET Assemblies can be placed in secured area called global assembly cache. This area  is equivalent to trusted class path of Java. Only system administrators can install or deinstall Assemblies from the global assembly cache. There is a place for downloaded or transient Assemblies called downloaded assembly cache. The Assemblies loaded from global assembly cache run outside the sandbox and have faster load time as well as enjoy more freedom to access file system resources. The Assemblies loaded from the downloaded cache area are subject to more security checks, therefore are slower to load and since they run inside the sandbox; enjoy much less privileges.
Assemblies manifests also contain information regarding sharing of code by different Applications and Application Domains.
To summarize, the Operating System can have multiple applications running simultaneously, each such application occupies a separate Win32 process and can contain multiple Application Domains. An Application Domain can be constructed from multiple assemblies.