An assembly is the functional unit of sharing and reuse in 
the Common Language Runtime. It provides the Common 
Language Runtime with the information it needs to be aware 
of type implementations. To the runtime, a type does not 
exist outside the context of an assembly. Assemblies are a 
fundamental part of the runtime.

In physical terms, an assembly is a collection of physical 
files that are owned by the assembly. These assemblies, 
called static assemblies, can include .NET Framework types 
(interfaces and classes) as well as resources for the 
assembly (bitmaps, JPEG files, resource files, etc.). In 
addition, the Common Language Runtime provides API's that 
script engines use to create dynamic assemblies when 
executing scripts. These assemblies are run directly and 
are never saved to disk, though you can save them to disk 
if you so choose. 

An assembly forms a logical unit of functionality, 
a “logical” dll. An assembly forms the fundamental unit of 
deployment, version control, reuse, activation scoping, and 
security permissions. Contained in an assembly is the 
assembly manifest, which contains all the metadata needed 
to specify the version requirements, security identity, and 
all information needed to define the scope of the assembly 
and resolve references to resources and classes.